This article needs to be wikified. Please do this by following our layout guide. (February 2022)

Sierra Entertainment was an American video game developer/publisher founded in the late 1970s by Ken and Roberta Williams.

The company is known for making the graphic adventure game genre, including the first game, Mystery House. It is also known for its adventure game series King's Quest, Space Quest, Police Quest, Gabriel Knight, Leisure Suit Larry, and Quest for Glory, as well as being the original publishers of Half-Life.

After 17 years as a lone company, Sierra was bought by CUC International in February 1996 to become part of CUC Software. However, CUC International was caught in an scandal in 1998. Therefore many of the original founders of Sierra including the Williamses left the company. Sierra remained as part of CUC Software as it was sold and renamed several times over the next few years; Sierra was dissolved as a company and reformed as a division of this group in 2004. The former CUC Software group was acquired by Vivendi and branded as Vivendi Games in 2006. The Sierra division continued to operate through Vivendi Games' merger with Activision to form Activision Blizzard in 2008. However, it was shut down later. The Sierra brand was revived by Activision in 2014 to re-release former Sierra games and some independently developed games.

Sierra Entertainment was founded in 1979 as On-Line Systems in Simi Valley, California, by Ken and Roberta Williams. Ken, a programmer for IBM, had planned to use the company to create business software for the TRS-80 and Apple II. Ken had brought a teletype terminal home one day in 1979, and while looking through the host system's catalog of programs, discovered the text adventure Colossal Cave Adventure. He encouraged Roberta to join him in playing it, and she was thrilled by the game; after Ken had brought an Apple II to their home, she played through other text adventures such as those by Scott Adams and Softape to study them. Dissatisfied with the text-only format, she realized that the graphics display capability of the Apple II could improve the adventure gaming experience. With Ken's help in some of the programming, Roberta designed Mystery House, inspired by the novel And Then There Were None and the board game Clue, using text commands and printout combined with graphics depicting the current setting.

On-Line Systems leased their first office space from Ponderosa Printing, a modest space in the back of a small town print shop.

Mystery House was released in 1980. It was a great success with 15,000 copies sold, earning 167,000 American dollars (equivalent to 525,539 American dollars in 2020). It is the first computer adventure game to have graphics, although made with crude, static, monochrome line drawings. The two decided to shift the company's focus to developing more graphical adventure games. Mystery House became the first of their Hi-Res Adventure series. The Hi-Res Adventure series continued with Mission Asteroid, which was released as Hi-Res Adventure #0 though being the second release. The next release, Wizard and the Princess, also known as Adventure in Serenia, is considered a prelude to the later King's Quest series in both story and concept. Through 1981 and 1982, more games were released in the series including Cranston Manor, Ulysses and the Golden Fleece, Time Zone, and The Dark Crystal. A simplified version of The Dark Crystal, intended for a younger audience, was written by Al Lowe and released as Gelfling Adventure.

On-Line Systems was renamed Sierra On-Line in 1982, and they moved to Oakhurst, California. The "Sierra" name was taken from the Sierra Nevada mountain range that Oakhurst was near, and its new logo had the image of a mountain reflecting that. By early 1984 InfoWorld estimated that Sierra was the world's 12th-largest microcomputer-software company, with $12.5 million in 1983 sales.

The company weathered the video game crash of 1983 by seeing only a 20% increase in sales, after analysts in 1982 had predicted a doubling in 1983 of the entire software market. The company had spent much of 1983 developing for a Commodore machine and the TI-99/4A which were both obsolete by the end of the year. Ken Williams was reportedly described as "bewildered by the pace at which computers come into and fall out of favor", and Williams said, "I've learned my lesson. I'm not moving until I understand the market better."

Many of Sierra's most well known series began in the 1980s. In 1983, Sierra On-Line was contacted by IBM to create a game for the new PCjr. IBM offered to fund the entire development and marketing of the game, paying royalties. Ken and Roberta Williams accepted and started on the project. Roberta Williams created a story featuring classic fairy-tale elements. Her game concept includes animated color graphics, a pseudo 3D-perspective where the main character is visible on the screen, a more competent text parser that understands advanced commands from the player, and music playing in the background through the PCjr sound hardware. For the game, a complete development system called Adventure Game Interpreter (AGI) was developed. In mid-1984, King's Quest: Quest for the Crown was released to much acclaim, beginning the King's Quest series.

Sierra On-Line expanded into a larger headquarters in the early-1980s.

While working to finish The Black Cauldron, programmers Mark Crowe and Scott Murphy began to plan for an adventure game of their own. After a simple demonstration to Ken Williams, he allowed them to start working on the full game, which was named Space Quest: The Sarien Encounter. The game was released in October 1986 as an instant success, spawning many sequels in the Space Quest series in the following years.

Al Lowe, who had been working at Sierra On-Line for many years, was asked by Ken Williams to write a modern version of Chuck Benton's Softporn Adventure from 1981, the only pure text adventure that the company had ever released. Leisure Suit Larry in the Land of the Lounge Lizards was a great hit and won the Software Publishers Association's Best Adventure Game award of 1987. It can be deduced that the game first became famous as an early example of software piracy, as Sierra sold many more hintbooks than actual copies of the game. A series of Leisure Suit Larry games followed.

Ken Williams befriended a retired highway patrol officer named Jim Walls and asked him to produce an adventure series based on a police theme. Walls proceeded to create Police Quest: In Pursuit of the Death Angel, which was released in 1987. Several sequels followed, and the series was touted for adherence to police protocol (relevant parts of which were explained in the games' manuals), and presenting some real-life situations encountered by Walls during his career as an officer.

Quest for Glory is a series of hybrid adventure/role-playing video games designed by Corey and Lori Ann Cole. The first game in the series, Quest for Glory: So You Want to Be a Hero, was released in 1989. The series combines humor, puzzle elements, themes and characters borrowed from various legends, puns, and memorable characters, creating a five-part series of the Sierra stable. Although the series was originally titled Hero's Quest, Sierra failed to trademark the name. Milton Bradley successfully trademarked an electronic version of their unrelated joint Games Workshop board game, HeroQuest, which forced Sierra to change the series' title to Quest for Glory. This decision caused all future games in the series (as well as newer releases of Hero's Quest I) to switch over to the new name.

In 1987, Sierra On-Line started to publish its own gaming magazine, about its upcoming games and interviews with the developers. The magazine was initially named The Sierra Newsletter, The Sierra News Magazine, and The Sierra/Dynamix Newsmagazine. However, since Sierra Club already published a magazine called Sierra Magazine, the name of the magazine published by Sierra On-Line was changed to InterAction in 1991. It was discontinued in 1999.

Sierra's Adventure Game Interpreter engine, introduced with King's Quest, was replaced in 1988 with Sierra's Creative Interpreter in King's Quest IV. The game was released under both engines, so those who had newer computers could use the new engine and better rendering technology.

• Books That Work; acquired in April 1997, folded into Sierra in February 1999.
• Bright Star Technology in Bellevue, Washington, U.S.; founded in 1980 and acquired in 1992.
• Berkeley Systems, purchased by CUC International in April 1997 and integrated into Sierra as an internal studio.
• Coktel Vision in Paris, France; founded in 1984, acquired in October 1993.
• Arion Software, acquired in 1995, absorbed into Sierra On-Line.^{[citation needed]}
• The Pixellite Group, founded in 1983; acquired in May 1995, absorbed into Sierra On-Line.^{[citation needed]}
• SubLogic, based in Champaign, Illinois; acquired in 1995; absorbed into Impressions Games.^{[citation needed]}
• Dynamix in Eugene, Oregon; founded in 1984, acquired in August 1990, and shut down in August 2001.
• Green Thumb Software in Boulder, Colorado, U.S.; acquired and absorbed in July 1995.
• Headgate Studios in Bountiful, Utah, U.S.; founded in 1992, acquired in April 1996, and sold to the original owner in 1999.
• Impressions Games in Cambridge, Massachusetts, U.S.; founded in 1989, acquired in 1995, and closed in May 2004.
• Synergistic Software; founded in 1978, acquired in 1996, and folded into Sierra in February 1999.
• Papyrus Design Group in Watertown, Massachusetts, U.S.; founded in 1987, acquired in 1995, and closed in May 2004.
• PyroTechnix; founded as Computer Presentation, acquired December 1997, and folded into Sierra in February 1999.
• Yosemite Entertainment in Oakhurst, California, U.S.; formed in 1998, and folded into Sierra in February 1999.

• Sierra Attractions; 1998–2001
• Sierra FX; 1998
• Sierra Home; 1996–2004
• Sierra Sports; February 1998–2000
• Sierra Studios; 1998–2001