Sierra Entertainment Inc. (formerly Sierra On-Line) was an American video-game developer and publisher founded in 1979 as On-Line Systems by Ken and Roberta Williams. Based in Oakhurst, California, the company was last owned by Activision, a subsidiary of Activision Blizzard.

Sierra is best known today for its multiple lines of seminal graphic adventure games started in the 1980s, many of which proved influential in the history of video games. The Sierra label was absorbed by its parent company. Some franchises (such as Crash Bandicoot and Spyro the Dragon) that were published by Sierra will be published by Activision, which also announced in 2008 that it may sell the Sierra brand.

Sierra Entertainment was founded in 1979 as On-Line Systems in Simi Valley, California, by Ken and Roberta Williams. Ken Williams, a programmer for IBM, bought an Apple II microcomputer with which he planned to use to develop a FORTRAN compiler for the Apple II. At the time, his wife Roberta Williams was playing text adventure games on the Apple II. Dissatisfied with the text-only format, she realized that the graphics display capability of the Apple II could enhance the adventure gaming experience. After initial success, On-Line Systems was renamed Sierra On-Line in 1982, and the company moved to Oakhurst, California.

Roberta Williams (born February 16, 1953) is an American video game designer and a co-founder of the Sierra On-Line company (later known as Sierra Entertainment). She is most famous for her pioneering work in graphical adventure games. She developed such titles as Mystery House, the King's Quest series and Phantasmagoria. She remains married to Ken Williams and retired from her career in 1999.

Roberta Williams, together with Ken Williams, is considered one of the most influential video game designers. Ars Technica stated that Roberta Williams was "one of the more iconic figures in adventure gaming".GameSpot once named her as the number ten in their listing of the Most Influential People in Computer Gaming of All Time for "pushing the envelope of graphic adventures" and being "especially proactive in creating games from a woman's point of view, and titles that appealed to the mainstream market, all the while integrating the latest technologies in graphics and sound wherever possible." In 2009, IGN placed the Williams at 23rd position in the list of top 100 game creators of all time, expressing hope that "maybe one day, we'll see the Williams again as well."