's biggest software company after more than 13 years at its helm. Here's a look at the ups and downs of his tenure:
— 1980: Ballmer joins the company co-founded by Bill Gates and Paul Allen in 1975.
— July 21, 1998: Widely seen as the successor to chairman and longtime friend Gates, Ballmer is named president of the software company. He'd previously led Microsoft's sales and marketing.
— Sept. 23, 1999: Ballmer warns that technology stocks are overvalued, including those of his own company, in public remarks that help accelerate a sharp sell-off on Wall Street. He blames a "gold rush" mentality for the stock price increases.
— Jan. 13, 2000: Gates promotes Ballmer to CEO of Microsoft while staying on as chairman and chief software architect.
— Nov. 15, 2001: Microsoft releases its Xbox video game system, one of the company's most successful products.
— March 26, 2004: At a conference for online advertisers, Ballmer says Microsoft's biggest mistake was failing to develop its own search engine, which caused it to fall behind rivals Google and Yahoo in the space. "That's probably the thing I feel worst about over the last few years," says Ballmer. Ballmer vows to put more money into research and development of search technology at Microsoft.
— May 4, 2006: Ballmer says MSN Search, Microsoft's search engine, is gaining steam against rivals, but still trails Google and Yahoo.
— November 13, 2006: Microsoft launches the Zune music player, its answer to Apple's iPod. The company discontinues the Zune five years later, in 2011.
— Feb. 1, 2008: Microsoft makes unsolicited offer to buy Yahoo for $44.6 billion. Microsoft withdraws its offer months later due to resistance from Yahoo.
— April 12, 2010: Microsoft unveils the Kin phone, an attempt to compete with Apple's iPhone. The phone was discontinued two months later.
— May 10, 2011: Microsoft announces it will buy Internet phone service Skype for $8.5 billion.
— Oct. 25, 2012: Microsoft holds a launch event in New York for Windows 8, a major overhaul of its ubiquitous computer operating system.
— May 7, 2013: Microsoft says it will retool Windows 8 to address complaints and confusion. Microsoft also discloses that it has sold more than 100 million Windows 8 licenses.
— July 11, 2013: Ballmer announces a sweeping restructuring of the company to cope with the quickening pace of technological change and competitive challenges presented by Apple and Google.
— July 18, 2013: Microsoft books a $900 million write-down for slashing the price of its Surface RT tablet. Its revenue and earnings come short of Wall Street forecasts in the April-June quarter.
— Aug. 23, 2013: Microsoft says Ballmer will retire in the next 12 months after 33 years with the company.