Bob Muglia, who has run Microsoft’s Server and Tools business since 2009, has agreed to leave the company during the summer as Microsoft continues its transition to a post-PC world of cloud computing.

Muglia has been at Microsoft for 23 years and had helped to build his division into a US$15 billion business unit for the company.

In an interoffice memo, CEO Steve Ballmer said Microsoft wants to build on its success and “move forward into the era of cloud computing.”

His memo went on to say that “The best time to think about change is when you are in a position of strength, and that’s where we are today with STB—leading the server business, successful with our developer tools, and poised to lead the rapidly emerging cloud future. Bob Muglia and I have been talking about the overall business and what is needed to accelerate our growth. In this context, I have decided that now is the time to put new leadership in place for STB…In conjunction with this leadership change, Bob has decided to leave Microsoft this summer. He will continue to actively run STB as I conduct an internal and external search for the new leader.”

Muglia was at the center of a minor dustup last year when he commented at the company’s PDC that HTML5—not Microsoft’s own Silverlight—was the best technology for cross-platform rich application development and delivery on mobile phones. Some took that to mean Microsoft was giving up on Silverlight, and it took the announcement of Silverlight 5 by corporate vice president Scott Guthrie to allay developer fears.