Call of Duty is a first-person shooter video game franchise. Starting out in 2003, it first focused on games set in World War II and was based primarily on Microsoft Windows. Over time, the series has seen games set in modern times, the midst of the Cold War, futuristic worlds, and outer space. Infinity Ward was the series' first developer, with Treyarch later becoming the second, creating a two-team development cycle. Sledgehammer Games later became the third developer in the cycle. Activision has served as the publisher for the series since its creation. Several spin-offs and handheld versions of titles have also been made by other developers. The most recent title, Call of Duty: Black Ops 4, was released on October 12, 2018.