Sonic X Shadow Generations

Sonic Generations is a platform game in which players control two variants of Sonic the Hedgehog: "Classic" or "Modern". Classic Sonic's gameplay is restricted side-scrolling gameplay similar to the original Sonic the Hedgehog games released in the 1990s, and requires him to use moves such as the Spin Dash and the Spin Attack. Modern Sonic's gameplay follows the gameplay style of Sonic Unleashed (2008), and Sonic Colors (2010), featuring an amalgamation of third-person and side-scrolling perspectives and techniques such as boosting and homing attacks. Both Sonics collect rings as a form of health, and power-ups such as elemental shields, invincibility, speed shoes, skateboards, or Wisps. Players can also customize each Sonic's abilities, and purchase additional ones at a Skill Shop.

The game's main hub is a realm called White Space, in which the player can explore, converse with other characters, and access the game's levels, each of which are derived from a previous entry in the series. The levels are split into three separate eras: the Genesis Era (focusing on levels from games for the Sega Genesis); the Dreamcast Era (focusing on levels from games for the Dreamcast, PlayStation 2, GameCube, and Xbox); and the Modern Era (focusing on levels from games for the PlayStation 3, Wii, Nintendo DS, and Xbox 360). Each level consists of two acts (one for each Sonic), and ten additional missions such as collecting a certain amount of rings or completing a level with the help of another character.

Left stranded in outer space following his previous defeat, Doctor Eggman comes across a mysterious and time-traveling entity known as the Time Eater; realizing its potential, Eggman schemes to use the creature to alter history for his own benefit. Eventually recruiting his past ("Classic") self to his cause, the duo convert the Time Eater into a cyborg-like abomination for their plot. The creature soon proceeds to attack ("Classic") Sonic as he travels through Green Hill during the prelude to the franchise's first game and interrupts a birthday celebration being held for the present ("Modern") Sonic by his friends, scattering them all through different time portals. Modern Sonic awakens in White Space and rescues Tails after traveling through Green Hill.

Modern Sonic and Tails eventually meet their Classic counterparts and form an alliance to restore their world. The duo travel through locations from their past they find in White Space, restoring them to normal. They rescue many of their friends, including Knuckles, Amy, Rouge, Cream, Blaze and the Chaotix, while battling past enemies for the Chaos Emeralds. Both Sonics eventually confront their respective Eggmen, who overpower them using the Time Eater.

Development of Sonic Generations began after the completion of Sonic Unleashed in 2008, alongside Sonic Colors, when developer Sonic Team began discussing possible ideas for a twentieth anniversary Sonic game. The console/PC version of the game was directed by Hiroshi Miyamoto, and Yoshinobu Uba was the Lead Game Designer. Producer Takashi Iizuka wanted a game that incorporated the best of Sonic's history, and offered more replay value compared to previous games. The team eventually decided to split the gameplay into two separate playstyles: one representing the original games and another representing more recent ones.

The team chose to develop on the high-definition (HD) PlayStation 3 and Xbox 360 consoles, wanting the game to re-imagine levels and aspects of previous entries with high-quality graphics. A port for Windows was also made; development was handled by Devil's Details. Dimps assisted with production of the Nintendo 3DS version. Sega considered making the 3DS version a port of the console versions, but instead chose to develop from scratch to make it unique and celebrate Sonic's portable history.

The console and PC versions of Sonic Generations received "generally favorable reviews", according to review aggregator Metacritic. The game was also successful commercially, selling 1.85 million copies worldwide across all platforms by March 31, 2012.

The game's presentation was well received. Nathan Meunier of GameSpot called the graphics arguably the best of the series, praising the detail put into recreating each level; this statement was echoed by Tim Turi of Game Informer. Jack DeVries and Brian Altano of IGN offered similar praise for the visuals, and described its remixed tracks as "cool" and its sound effects as "spot on". Tom Bramwell of Eurogamer felt Sonic Generations was a worthy tribute to the Sonic franchise, though he still considered the franchise's previous entry, Sonic Colors, to be better.

In 2017, GamesRadar named Sonic Generations the seventh best game of the Sonic series, and USgamer named it the tenth best. Following the reemergence of "Classic" Sonic, Sega would later begin to include him in other modern Sonic games, such as Sonic Dash (2013) and Sonic Runners (2015). The 2017 game Sonic Forces follows a similar premise to Generations, featuring both Classic and Modern iterations of Sonic as playable characters, and also uses an updated version of Generations' game engine. Some journalists initially believed Forces was a sequel to Generations, but Iizuka later clarified that it was a separate game.

Years after its release, the game has remained as a strong seller for Sega. On April 10, 2018, the Xbox 360 version was made backward compatible with the Xbox One, and was later extended to the Xbox Series X/S. An update to the Xbox 360 version in November 2021 added support for 4K visuals and an increased frame rate when played on Series X/S. Footage and music from Generations was used in the Sonic the Hedgehog pinball machine, released by Jersey Jack Pinball in 2026.