Half-Life 2 (stylized as HλLF-LIFE²) is a first-person shooter video game and the sequel to Half-Life (1998). Developed by Valve Corporation, it was released on November 16, 2004, following a five-year $40 million development, during which a substantial part of the project was leaked and distributed on the Internet. The game was developed alongside Valve's Steam software. It introduced the Source game engine and, because of Steam, was the first single-player video game to require online product activation.