Lead designer Randy Pitchford noted that he believed Gearbox was selected to develop Opposing Force because Valve, the creators of Half-Life, wanted to concentrate on their future projects. Opposing Force was the first expansion for Half-Life and was announced in April 1999. The game was developed by Gearbox Software and Valve and published by Sierra On-Line for Windows on November 19, 1999. Half-Life: Opposing Force is an expansion pack for the first-person shooter game Half-Life.