Welcome to the Cepheus Engine System Reference Document, which outlines the core rules used by a Classic Era Science Fiction 2D6-Based Open Gaming System. This Introduction provides you with an overview of what the Cepheus Engine is and how it works, while the later chapters show you how to create characters and embark on your own exciting science fiction adventures.

What Is Roleplaying?

If you are new to roleplaying, you might be asking yourself, "What is a roleplaying game?"

A roleplaying game (sometimes abbreviated as RPG) is a game in which players assume the roles of characters in a fictional setting. Players take responsibility for acting out these roles within a narrative, either through literal acting or through a process of structured decision-making or character development. Actions taken within many games succeed or fail according to a formal system of rules and guidelines, such as the Cepheus Engine rules.

A session of Cepheus Engine play is conducted through discussion. One player, the Referee, usually arbitrates any decision based on the rules and presents the setting of the game, while each of the other players plays the role of a single character. Together, the Referee and players create a story, much like the way many of us used to create stories playing games of make believe as young children, except with more dice and more structure.

What Is The Cepheus Engine?

The Cepheus Engine is simply a set of rules for conducting a game based on classic science fiction. Within its pages, you will find rules on creating characters, resolving tasks, fighting other creatures and engaging in huge space battles, building worlds, enjoy the risks of interstellar speculative trading, exploring new worlds, and a host of other diverse activities. Although no rules set can be complete, the Cepheus Engine attempts to provide enough rules to allow you to create almost any science fiction adventure.

The Cepheus Engine is inspired by classic science fiction games from the early days of roleplaying, and shares a lot of similarities with these gaming systems. Material from these older rules sets and those created under the Cepheus Engine are mutually compatible with only a minimum of adjustments required.

To play, you need a copy of the Cepheus Engine rules; several six-sided dice, available at game and hobby stores (you'll need two at a minimum, but more is better); and a pencil and some paper.

The Core Task Resolution System

The Cepheus Engine uses a core task resolution system to resolve actions. Whenever your character attempts any action with a chance of failure, do the following:

  1. Roll two six-sided dice (abbreviated 2d6).
  2. Add any relevant modifiers (for things like characteristics, skills, difficulty and circumstances).
  3. If the result equals or exceeds 8, the action succeeds. If the result is lower than 8, the action fails.

This simple system is used for nearly everything in the Cepheus Engine, with variations based on the modifiers added to a roll and the effects of success and failure.

The Referee

Within the Cepheus Engine rules, one of the players is asked to assume the mantle of Referee. The Referee is the person who will act as an organizer, officiant for questions regarding rules, arbitrator, and moderator for a Cepheus Engine gaming session, or an ongoing Cepheus Engine campaign. The role of the Referee is to weave the other participants' player-character stories together, control the non-player aspects of the game, create environments in which the players can interact, and solve any player disputes.

The Characters

A player character or playable character (PC) is a fictional character in a Cepheus Engine game whose actions are directly controlled by a player of the game rather than the rules of the game. The characters that are not controlled by a player are called non-player characters (NPCs). The actions of non-player characters are typically handled by the Referee.

Using the Cepheus Engine rules as guidelines, a player will create a character reflecting the sort of role they'd like to have in the game. The character is usually of a certain race (depending on those available with the Referee's universe) and possesses a unique combination of skills and levels of expertise. The attributes of a character is given as numerical values which can change as the gamer progresses, and characters grow and develop over the course of their adventures.


All characters have certain basic characteristics that define what they are capable of doing. These characteristics are Strength, Dexterity, Endurance, Intelligence, Education, and Social Standing. They each have a numeric characteristic score, averaging 7 for a normally capable human. Higher characteristic scores grant bonuses (+1 or more) on task resolution checks, while lower characteristic scores grant penalties (as low as -2). As part of creating your character, you decide how strong, smart, and tough your character is by choosing the appropriate characteristic scores. See Chapter 1: Character Creation for more information.


As a part of their development, characters follow certain life paths called careers. The Cepheus Engine provides a variety of career options for characters to pursue, which opens up opportunities for characters to gain skill levels, characteristic boosts and significant mustering out benefits before they enter into play. See Chapter 1: Character Creation for more information.


Skills represent training in a particular sort of task or knowledge, everything from acrobatic maneuvers to negotiation, piloting a starship, and programming a computer. Someone trained in athletics is able to climb faster and with more confidence than someone who isn't, for example. Skills are measured in levels, reflecting how much training a character has in the skill. When recording skills and their associated levels, the skill name is written first, followed by a hyphen, and then the number of levels that the character possesses in that skill. For example, Gambling-2 means that the character has two levels of Gambling skill.

Each skill level represents roughly two years' worth of education and training in that skill. For skills of an academic bent, you could equate that to an Associate's Degree (or similar two-year academic program) at level one, a Bachelor's Degree (or other four-year program) at level two, a Master's Degree at level three, a Doctorate at level four, and so on. Skill levels act as a bonus on task resolutions rolls when a character attempts an action related to a skill. As your character follows their career, they gain skills. See Chapter 2: Skills for more information on specific skills.

Game Play

Playing a session using the Cepheus Engine rules resembles verbally acting out chapters from a science fiction novel. Game sessions can last from short sessions of an hour or two, up to marathon games covering most of a weekend. The average game session tends to run three to five hours, however. Over the course of a session, the player-characters pursue their adventures. Some resolve quickly, within a single session, while others may take place in multiple acts spread out over several sessions.

Each adventure is like its very own story, consisting of a series of scenes that are explored during play. In general, most scenes are simply the players interacting with the non-player characters and the universe created by the Referee. The players describe their actions, and the Referee describes the results of those actions, in a back-and-forth exchange. Certain kinds of situations, such as personal or space combat, have more structure and more rules to help the players resolve their actions in ways reflecting the abilities of their characters.

Common Cepheus Engine Themes

On-going games using the Cepheus Engine rules are generally designed around a theme. Based in the classic era of science fiction, these rules lend themselves to games following certain common themes, which are detailed in the Common Cepheus Engine Themes table. This is not a complete list of the type of games that the Cepheus Engine is designed to handle; this is simply a general list of common themes for consideration.

Table: Common Cepheus Engine Themes
Theme Description
Colonial The adventurers are on the borders of explored space, helping to select a new world for a colony and then settle it. These campaigns allow a group to develop a single world extensively.
Commerce The adventurers live aboard a frontier trader or merchant trader, making a living through the transport of freight, passengers, speculative cargo and the occasional odd job. These campaigns heavily explore the trade and commerce rules.
Drifter The adventurers constantly move from place to place, without any fixed home or job, seeking employment through odd jobs as they explore the universe that the Referee has created. These campaigns are sometimes called "Sandbox Campaigns" because they explore an area that has been previously created and populated by the Referee.
Espionage The adventurers all belong to the same government or corporate intelligence agency, and make a living by going on spy missions against their enemies. These campaigns tend to be more episodic than other common campaign themes.
Exploration The adventurers serve aboard a survey vessel, making a living travelling through previously uncharted regions of space, exploring strange new worlds, and seeking out new life and new civilizations as they boldly go where no sophont has gone before. These campaigns make heavy use of the rules for generating worlds.
Mercenary The adventurers all belong to the same mercenary unit, making a living by participating in military actions in the hire of an interested party to the conflict. These campaigns make significant use of the personal combat rules, with a particular focus on large-scale military scenarios.
Political The adventurers are heavily involved in the social and political arena of an interstellar polity, dealing with diplomats, nobles and other factions with vested interests in controlling aspects of the government. These campaigns tend to focus on diplomacy and political intrigue.
Rebellion The adventurers are involved in a blossoming civil war, seeking either to preserve or overthrow the current government. These campaigns typically start with intrigue and end with action.

Die Rolls

Using the Cepheus Engine, when you have to make a die roll to resolve an action, it will typically follow the core system of a 2D6 roll plus modifiers versus a target of 8+ (read as eight or higher). This is called a check. You always want to roll high on a check. Rolling 12 before adding modifiers (where two sixes appear on the dice naturally) is not an automatic success, and rolling 2 before adding modifiers (where both dice show a one naturally) is not an automatic failure.

Difficulty and Effect

A check's Difficulty is a number set by the Referee that modifies your check result. When no Difficulty is given, the assumed Difficulty is +0. So, for a task with a Difficulty of +2, you must add +2 to the check result. You succeed on a total of 8 or better. The list of Difficulty ratings can be found in the Task Difficulty table.

Table: Task Difficulties
Difficulty DM
Simple +6
Easy +4
Routine +2
Average 0
Difficult -2
Very Difficult -4
Formidable -6

In some cases, the consequences of a check vary based on how much the check result is above or below the target of 8. The difference between the check result and the target of 8 is called the Effect. If the Effect is 6 or higher, the check is considered an Exceptional Success. When the Effect is -6 or lower, the check is considered an Exceptional Failure. See Chapter 2: Skills for more details.

Table: Degrees of Success
Effect Range Degree of Success
-6 or lower Exceptional failure
-1 to -5 Failure
0 to +5 Success
+6 or higher Exceptional success

Opposed Checks

If two characters are opposing each other directly in a task, then the character who obtains the highest Effect wins. For ties on opposed checks, the character with the highest relevant characteristic score wins. If the characters tie on characteristic scores, they reroll.

Trying Again

In general, you can try a check again if you fail, and keep trying indefinitely. Some tasks, however, have consequences for failure. For example, failing an Athletics check while climbing a cliff might mean you fall, which might make it difficult to try again. Some tasks can't be attempted again once a check has failed. For most tasks, once you've succeeded, additional successes are meaningless. (Once you've discovered a computer account's password using the Computer skill, for instance, there's no further benefit from additional Computer checks to determine the account's password.)

Circumstance Modifiers

Some circumstances make a check easier or harder, resulting in a bonus or penalty that is added to the check result. The Referee can alter the odds of success in two ways:

  • If a character has help, such as good tools, competent aids or other beneficial circumstances, he receives a +1 bonus to his skill check.
  • If a character is hampered, such as having defective tools, incompetent assistance or other negative circumstances, he receives a -1 penalty to his skill check.

Time and Checks

Outside of stressful situations such as combat, performing a particular task often takes a random amount of time, depending on circumstances. For most tasks, roll 1D6 and multiply it by the increment that the Referee provides for that action. For example, breaking a flimsy wooden door down may only require 1D6 seconds, while performing delicate surgery might take 1D6 hours. More information on time increments can be found in Chapter 2: Skills.

During combat, most actions are more refined to a finite period. Some of these actions are considered minor actions, while others are deemed significant actions. More information is provided in Chapter 5: Personal Combat.

Aiding Another

Sometimes characters work together and help each other out. In this case, one character (usually the one with the highest total of modifiers on the check) is considered the leader of the effort and makes the check normally, while each helper makes the same check. The Effect of a helper's check result can provide either a bonus (DM+1 with a successful result, DM+2 with an Exceptional Success) or a penalty (DM-1 with a failed result, DM-2 with an Exceptional Failure) to the leader's check result. In many cases, outside help isn't beneficial, or only a limited number of helpers can aid someone at once. The Referee limits aid as appropriate for the task and conditions.

Types of Checks

There are two main types of checks: skill checks and characteristic checks.

Skill Checks

A skill check determines what you can accomplish with a particular skill (sometimes whether you're trained in that skill or not). It is a roll of 2D6, modified by your levels in the skill, the skill's key characteristic score modifier, and the Difficulty, against a target of 8 or higher. Skill checks sometimes have gradations of success and failure based on the Effect of your check result (how much higher or lower your check result is when compared to the target of 8). When making a skill check, if a character does not have any levels in the required skill, then he suffers a –3 penalty for being unskilled.

Attack Rolls

An attack roll determines whether or not you hit an opponent in combat. An attack roll is essentially a skill check, using your skill levels in combat skills as modifiers.

Characteristic Checks

A characteristic check is like a skill check, but measures raw ability, like strength, endurance, or intelligence. These checks are used when the task is one not covered by an obvious skill, or where the character's innate abilities are the most important influence on the result. To make a Characteristic check, you must roll 2D6 and add the appropriate characteristic score modifier. Characteristic checks tend to be all or nothing (you can either accomplish the task or you can't), although there are sometimes gradations of success or failure.

The Combat Round

When things really start happening in a Cepheus Engine game, time is broken down into six-second segments called rounds, or combat rounds, since they're most often used in fights. A round isn't very much time, just long enough for a character to do something. The types of actions your character can perform during a round are significant actions, minor actions, extended actions, free actions, and reactions. During a round you can do one of the following:

  • Initiate or continue an extended action.
  • Take a significant action and a minor action.
  • Take three minor actions, and forego taking a significant action this turn.

You can perform as many free actions and reactions in a round as you wish, although the Referee may choose to limit them to a reasonable number to keep the game moving. See Chapter 5: Personal Combat for more information.

Pseudo-Hexadecimal Notation

The Cepheus Engine uses a form of pseudo-hexadecimal notation as a type of shorthand in noting specific values of characteristic scores, world statistics, drive type designations and similar design elements. The pseudohexadecimal notation proceeds as normal for values from 0 to 15, but extends beyond F for 15, with G for 16, etc. The Cepheus Engine skips the use of the letters I and O, because they might be mistaken for the numbers 1 and 0. The Pseudo-Hexadecimal Notation table provides a quick reference for converting values for use in Cepheus Engine.

Table: Pseudo-Hexadecimal Notation
Actual Value PseudoHex Actual Value PseudoHex Actual Value PseudoHex
0 0 12 C 24 Q
1 1 13 D 25 R
2 2 14 E 26 S
3 3 15 F 27 T
4 4 16 G 28 U
5 5 17 H 29 V
6 6 18 J 30 W
7 7 19 K 31 X
8 8 20 L 32 Y
9 9 21 M 33 Z
10 A 22 N
11 B 23 P

Important Terms

The Cepheus Engine uses certain words and abbreviations throughout the rules system. In order properly understand the Cepheus Engine rules, both players and Referees should become familiar with these terms. The following words, phrases and abbreviations are commonly used in The Cepheus Engine:

Two six-sided dice, used to resolve all actions in the Cepheus Engine.
A character activity. There are significant actions, minor actions, extended actions, free actions, and reactions.
A story created by the Referee and players, comprised of a series of related scenes or encounters.
Attack bonus
A modifier used to measure a character's combat skill.
Attack roll
A skill check used to determine whether an attack hits.
Any of numerous actions intended to harm, disable, or neutralize an opponent.
A positive modifier to a die roll.
A series of linked adventures.
A fictional individual in the game. The players control characters, while the Referee controls nonplayer characters.
Characteristic modifier
A modifier determined by the value of the characteristic score, applied as a bonus or penalty to checks as needed.
Characteristic score
One of the six basic character traits -- Strength (Str), Dexterity (Dex), Endurance (End), Intelligence (Int), Education (Edu), and Social Standing (Soc).
Check (or Throw)
A method of deciding the result of a character's action. Checks are based on a relevant ability, skill, or other trait. To make a check, roll 2D6 and add any relevant modifiers. If the check result equals or exceeds a target of 8 or the result of an opponent's check, it succeeds.
An astrographical term indicating the direction of Sagittarius A* (pronounced "Sagittarius A-star", standard abbreviation Sgr A*), a bright and very compact astronomical radio source at the center of the Milky Way galaxy.
Credit (Cr)
The primary unit of currency used in the Cepheus Engine. For very large amounts of money, the kilocredit (KCr) represents one thousand credits and the megacredit (MCr) represents one million credits.
Critical hit (crit)
An attack inflicting extra damage. Critical hits are only involved in vehicular and space combat.
A special die roll generated by rolling two six-sided dice of different colors (or rolling one die twice, noting each number rolled), multiplying the first die by 10 and then adding the second die, to create a number between 11 and 66.
Damage bonus
A modifier used to determine the damage of an attack.
Harm caused to a character by injury, illness, or some other source.
Dice modifier (DM)
A modifier applied to a check.
Die (plural is dice)
A small polyhedron, typically a cube, with each side having a different number on it, ranging from one to the number of sides of the polyhedron, thrown and used in gambling and other games involving chance. The Cepheus Engine uses six-sided dice exclusively to create random results during play.
A modifier applied to a check that is assigned by the Referee, reflecting the relative ease or difficulty of a given action.
Dominant race
A sentient species capable of interstellar travel via jump drive technology, and that has used that technology to expand their presence over a significant region of space. Dominant Races typically control a fairly expansive interstellar government, and have settlements and significant populations more than ten parsecs away from their planet of origin.
Near death and unconscious. A dying character can take no actions.
The difference between a check result and the target of 8 (i.e. how much higher or lower the result is).
An unexpected or casual meeting with someone or something. A large part of the Referee's job is the administration of encounters.
Explorer's Society
A private interstellar travel service which maintains exclusive resorts and facilities at various starports. Several interstellar organizations provide membership to the Explorer's Society as a reward for outstanding service.
Exceptional failure
Any check that fails by 6 or greater (i.e. has an Effect of -6 or worse).
Exceptional success
Any check that succeeds by 6 or greater (i.e. has an Effect of +6 or better).
Extended action
An action in combat that takes longer than a single combat round to complete.
Free action
An extremely fast activity, requiring very little time and effort.
1)The world that serves as a character's place of origin, usually the world on which the character was raised and which had the most impact on their development during their pre-adult life. 2) The world of origin for an alien species, i.e. Earth is the homeworld for the human species
A form of faster-than-light movement using Jump drives, which always takes one week to travel a number of parsecs equal to its Jump rating and consumes a vast amount of fuel.
Jump point
A point in space more than 100 diameters out from any nearby celestial body, chosen by a navigator as the point from which a ship will enter into Jump space.
Jump space
The alternate dimension through which starships travel when transitioning from one point in normal space to another.
Lesser race
A sentient species that has not developed jump drive technology on its own. While individuals of the Lesser races can and do engage in interstellar travel, settled populations of any given Lesser Race are rarely encountered further than ten parsecs from their homeworld.
Lethal damage
Damage that can potentially disable or kill a target.
The primary world of a star system; the world represented by the UWP in a list of worlds for a given region of space.
Melee attack
A physical attack in close combat.
Melee weapon
A handheld weapon designed for close combat.
Minor action
An action intended to move a distance or to manipulate or move an object. You can take up to three minor actions per round, at the loss of a significant action.
A mishap caused by an inaccurate jump, which results in jumping to a random location with damage to the vessel and potentially the crew. Common causes for misjumps include bad Jump plots, damaged Jump drives, diverting energy into the Jump drive improperly, jumping from within the hundred-diameter limit and using unrefined fuel.
Any bonus or penalty applied to a die roll.
A natural result on a roll or check is the actual number appearing on the die, not the modified result obtained by adding bonuses or subtracting penalties.
Non-lethal damage
Damage that can potentially stun or knock out a target, but does no permanent harm.
Non-player character (NPC)
A character controlled by the Referee (as opposed to a character controlled by a player).
A spaceship without a jump drive, and thus incapable of interstellar travel on its own.
A non-player character who gives financial or other support to a person, organization, cause, or activity. Referees often use patrons as a tool to attempt to engage player characters in adventures.
A negative modifier to a die roll.
Player character (PC)
A character controlled by a player, one of the protagonists of an adventure or campaign.
A character with psionic abilities.
Ranged attack
Any attack made at a distance.
Ranged weapon
A projectile or thrown weapon designed for attacking at a distance.
An action taken in response to the action of another. You can perform as many reactions as you want per round in Personal Combat, but the number of Reactions that a vessel can undertake is based on Initiviative in Space Combat.
The player who portrays characters not controlled by the other players, arbitrates the rules, and makes up the story and setting for the game.
An astrographical term indicating the direction opposite of Coreward, or directly away from the radio source of Sagittarius A*.
A six-second unit of game time used to manage actions, usually in combat.
Significant action
An action intended to do something within about 3 seconds. You can perform a single significant action per round, or forego it to perform a total of three minor actions.
Seriously wounded
If you have lost at least one point from all three of your physical characteristics, you are considered seriously wounded. When conscious, you cannot move except to hobble or crawl along at 1.5 meters per combat round. You also lose your minor action in combat. You can only regain characteristic points equal to your Endurance DM per day of rest through natural healing. You require surgery.
An ability to perform a set action, such as navigating a starship, operating a rifle, or programming a computer. Skills are attained in levels (Navigation-1, Computer-2, etc); the higher the level of a skill, the more expertise a character has in that area. Many different individual skills are available to characters.
Small Craft
A vessel under 100 tons, capable of interplanetary travel.
A sentient being with a base reasoning capacity roughly equivalent to or greater than that of an average human being.
An astrographical term indicating the direction of the galaxy's rotation.
Standard Day
A unit of time that is 24 hours long.
Standard Year
A unit of time that is 365 Standard Days in length.
A port where interstellar and interplanetary vessels load or unload, especially one where customs officers are stationed.
A spaceship with a jump drive, capable of interstellar travel on its own.
Target (also subject)
The intended recipient of an attack, action, or effect.
An astrographical term indicating the direction opposite of the galaxy's rotation.
Having knowledge of, and therefore levels in, a skill.
Unarmed attack
A melee attack made with no weapon.
The setting presented by a Referee, in which characters play out adventures and campaigns. A Referee may use a published setting for their adventures, or create their own with the Cepheus Engine rules.
Having no ranks in a skill. Some skills cannot be used untrained. Unskilled skill checks suffer a DM-3 penalty.
General term used to starships, small craft, or vehicles as a general inclusive group. Most commonly, it refers to any vehicle or ship capable of interplanetary or interstellar travel.
A generic term in the Cepheus Engine for any inhabitable celestial body or similar location represented by a Universal World Profile.