FORTUNE — Microsoft co-founder Bill Gates led the company from its inception through 1999. After that, Steve Ballmer, the 30th employee hired by a company that now has over 94,000, took over as CEO. Over the course of its storied history, one of the most important companies in the world has — with help from co-founder Paul Allen — been led by just two men. That’s a lot of legacy.

This year may be one of the company’s most significant ever. It is releasing a new version of its mainline operating system alongside new tablets. Steve Ballmer, 56, doesn’t seem to be going anywhere anytime soon. Still, who might replace him eventually? Here are three possibilities:

Andrew Lees, president
Lees has been at the company since 1990. However, prior to his current gig, he ran the company’s ailing Windows Phone division — not the best resume booster. Plusses including a keen understanding of Microsoft’s most important businesses. Before phones, Lees worked on the successful server division

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Craig Mundie, chief research and strategy officer
Mundie was instrumental in helping Microsoft become the company it is today. He played a central role in strategy, for instance, helping craft Microsoft’s vision across Windows, handhelds, and the Xbox gaming console. But at 63, the chances of him taking over the company seem slim. Still, Mundie has been a central player in Microsoft’s recent successes and seems to understand the long-term trends that will define the company. His “big ideas” style of leadership might be right for a company with so many complex divisions.

Steven Sinofsky, president of the Windows and Windows Live
Sinofsky might be the obvious replacement should the company needs a rapid change. For stalwarts scared of changing Microsoft’s secret sauce for Windows, that’s a good thing. But for those who are rightfully more concerned with someone that can manage Microsoft’s many moving parts, his sole focus on Windows is a problem. Windows is still central to Microsoft’s business, and if anyone knows about that operating system, it’s Sinofsky. And yet, Windows isn’t everything anymore. And depending on how Windows 8 shakes out, focusing too heavily on it could be a real issue for Microsoft.