Canon

'Canon' refers to the body of works considered 'officially' sanctioned or otherwise 'genuine'. In the entertainment industry, where ownership of a property is sometimes shared between companies with various interests, be they financial or creative, the characters, locations and events of such a property may come to be used in works over which the original authors have limited creative control. Additionally, even later works by the original authors may contradict facts established in earlier works, leaving an ambiguity as to which work represents the canonical truth.

The Half-Life Canon

The following games are officially considered by Valve (the 'official' source of the Half-Life Universe) to be canonical:

• Half-Life
• Half-Life: Opposing Force
• Half-Life: Blue Shift
• Half-Life: Decay
• Half-Life 2
• Half-Life 2: Episode One
• Half-Life 2: Episode Two
• Half-Life 2: Episode Three
• Portal

Additionally, the following games (though not officially sanctioned) do not contradict the official storyline and are potentially canonical:

• Half-Life 2: Lost Coast

However, these are not the only games to exist that feature elements from the Half-Life universe or claim to take place there. The following games are technically Half-Life properties, yet they contradict significant plot points and are thus not considered canonical:

• Codename: Gordon
• Half-Life: Uplink

Additionally, an early promotional screenshot of the original version of the Team Fortress sequel, Brotherhood of Arms featured the G-Man potentially suggesting that the game, released in such a form, may have been set in the Half-Life universe.

Gearbox's Expansions

By far the greatest contribution made outside of Valve to the Half-Life Universe has been the output of Gearbox. This includes not only the expansions, Opposing Force, Blue Shift and Decay but also console ports of Half-Life for Playstation 2 and Dreamcast. Because of their substantial outside input, there has been considerable debate as to the canonical status of these titles and the significant additions they make. The official line from Valve is that the events of the Gearbox expansions took place, and were overseen by series writer Marc Laidlaw. However, the extent to which Valve had control over finer details is unknown. Despite their apparent acceptance of the Gearbox expansions, Valve have so far avoided mentioning any of the significant additions that were made during later titles in the Half-Life series. The one possible exception is that of Barney Calhoun, who starred in Half-Life: Blue Shift. However, as the character makes no specific reference to the situations and people involved in his escape from Black Mesa (and meanwhile makes reference to promises of Beer made by other security guards), it isn't actually 100% certain that Valve's Barney Calhoun is Gearbox's Barney Calhoun.

Mods and Other Media

No user-created Modification has ever been accepted as part of the Half-Life canon (and it is unlikely that such a work ever will be). An important example is that of Half-Life: Absolute Redemption, a Single-Player modification eventually selected for inclusion in Sierra's American 'Half-Life Platinum' bundle. Though given the distinction of a commercial release, the mod is still not considered canonical and as a sequel to Half-Life, it contradicts the genuine sequels on a number of significant counts.

Other products bearing the Half-Life name exist in other formats. Particularly, in print in the form of Prima authored strategy guides and the Half-Life 2 making of artbook, Raising the Bar. The former are largely considered to be canonical as they are overseen by Valve themselves. Though also overseen by Valve, the content of Raising the Bar is conceptual and cannot be entirely trusted. Nevertheless, certain details in the book may be taken as viable explanations for unexplained phenomena found in the game series itself.