S.T.A.L.K.E.R.: Lost Alpha is a standalone Modification based upon the S.T.A.L.K.E.R.: Shadow of Chernobyl game originally developed by GSC.

As a standalone game, the most obvious difference from Shadow of Chernobyl is the sheer immensity of the Lost Alpha Zone in comparison.

Many previously cut levels have been re-introduced such as Darkscape and Dead City where salient plot-lines are executed in a considerably more detailed, and arguably, more coherent main quest.

While the weaponry and armour remain largely unchanged from the Base Game, upgrading has been engineered into the game and behaves similarly to these functions seen in the other games: S.T.A.L.K.E.R.: Clear Sky and S.T.A.L.K.E.R.: Call of Pripyat. Technicians are, however, in short supply. Weaponry has been rebalanced in terms of firepower (usually upwards for small arms and low-end weaponry) with the result that players will probably now wish to maintain suitable sidearms for the duration of the game.

Since the Lost Alpha Zone is so large and diverse, vehicles are re-introduced to facilitate Zonal travel. Many are available within the levels for free, others may be purchased from the Barkeep.

Just about every mutant that previously appeared in an earlier Build has been re-introduced and possibly some that are unique in their own right.

In addition to an enlarged Lost Alpha Zone, there are also many more underground locations. Some only become apparent as a result of side-missions or main-mission tasks but they are all expansions of the Base Game.

Where the affiliation of the player's actor was well demarcated in Shadow of Chernobyl and became more variable in Clear Sky, players will find that in this Mod that their actor may actively use camouflage by mimicking other factions through the use of appropriate dress. Although Faction Wars are not a feature of the game, the disposition of other Factions to the player varies with the progression of the storyline. Depending upon the player's decisions this will be most notable with the Military and the Monolith factions.