What order should I play the Half-life series in?

The HL2 episodes are not an accessory part of the story, but actual sequels. They do contain plot changing events. The same is not true for the Half Life "One" series, as far as I know.

Thus:

1. Half Life
2. Half Life 2
3. Half Life 2 Episode 1
4. Half Life 2 Episode 2

As for Portal -- it does contain some side references to corporate entities in the Half Life universe, but as it is that's more flavor than actual plot.

The Source versions are a remake of the original games on the engine used by HL2. Half Life: Source is pretty much identical to Half Life.

The recommended playing order is pretty much the release order:

• Half Life (Or Half Life: Source)
• Half Life: Opposing Force
• Half Life: Blue Shift
• Half Life: Decay (skippable - PS2 only)
• Half Life 2
• Half Life 2: Episode One
• Half Life 2: Episode Two

Half Life 2: Lost Coast is just a tech demo for graphics options that went into later Source games. If you want to be quick, you can skip everything but Half Life and Half Life 2.

Portal is also tangentially related to the Half Life plot, but isn't too important as it doesn't have much story of its own. Portal 2 may change this however.

As already pointed out, the most logical order is to play the games by their initial release dates. This is assuming you want to follow the Half Life story from start to finish.

1. Half-Life - You take control of Gordon Freeman in the Black Mesa facility
2. Half-Life: Opposing Forces - You play as one of the marines during the events of the original game
3. Half-Life: Blue-Shift - You play as a security guard during the events of the original game
4. Half-Life 2 - Years later you take control of Gordon Freeman in a new story
5. Half-Life 2 Episode 1 - Continue as Gordon Freeman after the events of Half-Life 2
6. Half-Life 2 Episode 2 - Continue as Gordon Freeman after the events of Half-Life 2 Episode 1

Someone has already mentioned "Half-Life Decay". This is a coop game mode available on the Half-Life PS2 release. It takes place during the events of the first game. It is possible to play this on the PC (as someone has managed to port it onto the PC).

Lost Coast is a very short demo. It takes part in the realms of Half Life 2, but from memory the location doesn't appear in the full game.

Half-Life also has a rich mod community. One of the stand out mods would be the Black Mesa project - whereby Half Life 1 is being remade into the Source engine, used to make Half Life 2. The difference between this mod and the Half-Life: Source release is that the graphics are being completely polished.

Depending on your tolerance, or appreciation, for older games, it is feasible to completely skip Half Life 1, or at least the expansions. They were great games but are still old games. Not to say Half Life 2 isn't an old game.

Will you miss anything not playing Half-Life 1? Not really. The story in Half-Life 2 is so different. A different time, a different location, different enemies... If Half-Life 1 never existed, Half-Life 2 would still be a great game and story.

If you play Half-Life 2 then the episodes are worth going through as they immediately follow events from the main game. There is also a cliff-hanger at the end of Episode 2. Episode 3 never appeared and people have been waiting a generation for the next installment.

Portal has been mentioned but the game is not like Half-Life. Don't assume that you MUST play Portal 1 and/or 2 because you played Half-Life.

There appears to be a lot of opinion and speculation generated from this question, so I am going to use the Wikipedia listing of the chronological Half-Life games, in order to give a rough overview of each game in the series. See below on my thoughts for deciding if you should play certain titles.

Which Games Should I Play?

I can't really tell you. The Half-Life series is so broad and diverse that it spans many releases, some official, some not. I have provided meta-reviews, where applicable, to help provide a general consensus of quality; However, due to different opinion, you might very well enjoy titles that were effectively shunned by the general community.

For this reason, I would mostly advise you to do your own research. Watch somebody play on YouTube. Read your own reviews on the game. In some cases, you can purchase packs of older titles at a good price, so you might consider buying a set of games and making your decision within the first hour or so of game-play.

There are some absolutes I can provide, that is, what games you should play for the purpose of story. Please keep in mind that some games are re-makes, in one way or another. In such cases, I will only list the original version. It is again up to you, personally, to decide whether you would have a greater experience replacing said title for the newer version, or if you in fact wish to play the very first release.

I am very interested in the story, including minor plot.

The following titles provide story, and are ordered in approximate order. There are several games the occur around each other, from alternate points of view, which can easily be played in any order. Furthermore, there are several titles that do not provide any form of story, which will be omitted.

Half-Life -> Half-Life: Opposing Force -> Half-Life: Blue Shift -> Half-Life 2 -> Half-Life 2: Episode One -> Half-Life 2: Episode Two

I am only interested in major story, and do not wish to involve minor plot.

The following title order will provide you with the main Half-Life plot. They are the main releases, with the episode releases intended to replace the expected long waiting time for Valve to develop an entirely new game.

Half-Life -> Half-Life 2 -> Half-Life 2: Episode One -> Half-Life 2: Episode Two.

The Games

Half-Life (also see Black Mesa)

• Genre: Single-Player First Person Shooter
• Original Release Date: 1998
• Developer:
  • Valve Corporation (original release)
  • Gearbox Software (PlayStation 2 port)
• Platform: Windows, OS X, Linux and PlayStation 2
• Engine: GoldSrc
• Metacritic Score: 96 on PC, 87 on PlayStation 2
• GameRankings Score: 94 on PC, 83 on PlayStation 2
• Notes: The first game in the Half-Life series, this game will take you back to where it all began, placing you in control of main protagonist Gordon Freeman. According to the developer, Half-Life has won over 50 Game of the Year awards, and sold over 20 million copies.

Half-Life: Source

• Genre: Single-Player First Person Shooter
• Original Release Date: 2004
• Developer: Valve Corporation
• Platform: Windows, OS X, Linux
• Engine: Source
• Metacritic Score: no score
• GameRankings Score: no score
• Notes: Half-Life, remade using the Source engine. It is worth noting that this particular release was criticised for not being a complete remaster, which lead to the development of Black Mesa.

Half-Life Deathmatch: Source

• Genre: Multi-Player First Person Shooter
• Original Release Date: 2006
• Developer: Valve Corporation
• Platform: Windows, OS X, Linux
• Engine: Source
• Metacritic Score: no score
• GameRankings Score: no score
• Notes: The multi-player component of the original Half-Life, remade using the Source engine. Much like Half-Life 2 Deathmatch, this game is simply a multiplayer game, and does not have any story.

Half-Life: Opposing Force

• Genre: Single-Player First Person Shooter
• Original Release Date: 1999
• Developer: Gearbox Software (under direction of Valve)
• Platform: Windows, OS X and Linux
• Engine: GoldSrc
• Metacritic Score: no score
• GameRankings Score: 85 on PC
• Notes: Valve gave the development reigns to Gearbox, for this title, in order to focus on other projects. It is played from the perspective of Adrian Shephard,

  A member of the marine squad sent in to "clean up" Black Mesa in Half-Life.

Half-Life: Blue Shift

• Genre: Single-Player First Person Shooter w/ Multiplayer
• Original Release Date: 2001
• Developer: Gearbox Software (under direction of Valve)
• Platform: Windows, OS X, Linux and PlayStation 2
• Engine: GoldSrc
• Metacritic Score: 71 on PC
• GameRankings Score: 67 on PC
• Notes: Another title developed by Gearbox, in this title, players take control of Barney Calhoun.

  Barney Calhoun is a security guard in Black Mesa, and a central character in Half-Life 2.

Another feature this game brought was the inclusion of high-definition models and textures, which would also upgrade Half-Life and Half-Life: Opposing Force.

Half Life: Decay

• Genre: 2 Player Co-op First Person Shooter
• Original Release Date: 2001
• Developer: Gearbox Software (under direction of Valve)
• Platform: PlayStation 2
• Engine: GoldSrc
• Metacritic Score: no listing
• GameRankings Score: no listing
• Notes: Designed as a 2 player co-op, for release with the PlayStation 2 version of Half-Life, Decay puts the players in control of two scientists inside Black Mesa. While only officially released for PlayStation 2, there also appears to be an unofficial port to PC.

Black Mesa

• Genre: Single Player First Person Shooter
• Original Release Date: 2005
• Developer: Valve
• Platform: PC
• Engine: Source
• Metacritic Score: no listing
• GameRankings Score: no listing
• Notes: Black Mesa is designed as a remake of Half-Life using the Source engine. This game is currently in early access. This means that if you pay for the game, now, you can play it, now. But it is not, in any way, a finished product. This is the main reason for no Metacritic or GameRankings score, although in house, Steam have received "overwhelmingly positive" reviews from the players, at the time of writing this. You can ask questions about this game, since it is semi-available to the public. But be aware that only a select group of people will be able to answer them.

Half-Life 2 (see Lost Coast, included with HL2)

• Genre: Single-Player First Person Shooter
• Original Release Date: 2004
• Developer: Valve
• Platform: Windows, OS X, Linux, Xbox, Xbox 360 and Shield Portable
• Engine: Source
• Metacritic Score: 96 on PC
• GameRankings Score: 95 on PC
• Notes: Sequel to Half-Life, this is the first game to put players back in control of Gordon Freeman. It is critically acclaimed, and has sold over 12 million copies.

Half-Life 2: Deathmatch

• Genre: Multi-player First Person Shooter
• Original Release Date: 2004
• Developer: Valve
• Platform: Windows, OS X and Linux
• Engine: Source
• Metacritic Score: 74 on PC
• GameRankings Score: no score
• Notes: Half-Life 2: Deathmatch is simply a stand alone multiplayer component for Half-Life 2. It allows you to compete, online, but does not involve any story.

Half-Life 2: Lost Coast

• Genre: First Person Shooter
• Original Release Date: 2005
• Developer: Valve
• Platform: Windows, OS X and Linux
• Engine: Source
• Metacritic Score: no score
• GameRankings Score: no score
• Notes: Lost Coast is a technology demonstration. It was created to showcase the high-dynamic-range rendering capabilities of the Source engine. While this was released separate to Half-Life 2, it should be considered "DLC" or an additional level, and is included with the purchase of Half-Life 2.

Half-Life 2: Episode One

• Genre: First Person Shooter
• Original Release Date: 2006
• Developer: Valve
• Platform: Windows, OS X, Linux, Xbox 360 and PlayStation 3
• Engine: Source
• Metacritic Score: 87 on PC
• GameRankings Score: 86 on PC
• Notes: Half Life 2: Episode One is the first episodic sequel to Half-Life 2. Given the long wait between Half-Life and Half-Life 2, Valve have decided to focus on releasing episodes, in effort to reduce the long waiting period between game releases. The player takes control of Gordon Freeman, again, with a greater focus on developing the other characters.

Half-Life 2: Episode Two

• Genre: First Person Shooter
• Original Release Date: 2007
• Developer: Valve
• Platform: Windows, OS X, Linux, Xbox 360 and PlayStation 3
• Engine: Source
• Metacritic Score: 90 on PC
• GameRankings Score: 91 on PC
• Notes: Half Life 2: Episode Two is the second episodic sequel to Half-Life 2, and direct sequel to Episode One. It was developed alongside Episode One, to give the two titles a greater sense of immersion.

Half-Life 2: Episode Three

The internet is filled with rumors concerning Half-Life 2: Episode Three. Is it really coming? Have Valve ditched the "episodic structure", and instead, working on Half-Life 3? You will see "episode three" listed within the game series, but be aware that most of it, at present, is speculation. When the game is released, this answer can be edited to include the appropriate information. I would speculate that, given the amount of time since Episode 2 compared to the time it took to develop Episode One and Episode Twp, and the expectation that an entire Half Life game would take a lot more time, we should be expecting Half Life 3 rather than another episode.

Further Information

• There seems to be some users who advise "don't play that game, it was not made by Valve". All Half-Life games listed above were made under direction of Valve. They gave control of a majority of original Half-Life titles to Gearbox, so they could work on other titles. Gearbox is the company behind Borderlands and Duke Nukem Forever, two very conflicting examples of widely-accepted first-person shooters, so my best advice is don't knock it unless you have tried it.
• If you are unsure about playing a game, look up the reviews. You should be playing the game because you think you will enjoy it, not because a group of fanboys say its bad. Just because I like apples, does not mean you have to. Thats what makes us human.
• You can find any missed story collected together on the Half-Life Wikia. I have deliberately used the scores from MetaCritic and GameRankings, as they attempt to give an aggregated average across many other reviews. If you are the sort of person that looks at what the reviews have to say, before playing, go to a website you respect the opinion of, and see what they have to say. This is not the place for personal opinions.
• The portal series is a series in its own right, but acts as a sort of "spin-off" to the Half-Life series. While not directly related, one could assume they are set in the same fictional universe, with references to Black Mesa lightly used in-game.

Furthermore, it is possible to "replace" the Gravity Gun in Half-Life 2 with the Portal Gun from Portal, with some easy manipulation on the user-end.