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Half-Life: Opposing Force is an expansion pack for the first-person shooter game Half-Life (1998). It was developed by Gearbox Software and published by Sierra On-Line for Windows on November 19, 1999. Opposing Force was the first expansion for Half-Life and was announced in April 1999. Lead designer Randy Pitchford believed Gearbox was selected to develop Opposing Force because Valve, the creators of Half-Life, wanted to concentrate on their future projects. Over the course of development, Gearbox brought in a variety of talent from other areas of the video games industry to help bolster various aspects of design.