Quite like its predecessors, the core gameplay of GTA IV consists of giving the player a large, open world environment in which to move around freely. On foot, the player character can walk, run, jump, climb over obstacles and swim, as well as use weapons and perform basic hand-to-hand combat. Players can steal and drive a variety of vehicles, including automobiles, boats, helicopters, and motorcycles. Grand Theft Auto IV takes advantage of Natural Motion's Euphoria engine, which combines artificial intelligence, bio-mechanics and physics to make open, non-linear environments that allow players to explore and choose how they wish to play the game. Although completing most of the storyline missions are necessary to progress through the game and unlock certain content and parts of the city, they are not required, as players can complete them at their own leisure. When not attempting a storyline mission, players can free-roam, giving them the ability to do activities. Side missions such as locating and destroying criminals in the police car database or participating in street races can keep the player occupied for hours. The player can also swim just like in the previous entry of the game, this feature was missing from older games.
