Microsoft  Silverlight Release History

Silverlight 5
Silverlight 5 Build 5.1.10411.0 Released May 8, 2012
Silverlight 5 Build 5.0.61118.0 Released December 9, 2011
Silverlight 5 RC Build 5.0.60818.0 Released September 1, 2011
Silverlight 5 Beta Build 5.0.60401.0 Released April 13, 2011

Silverlight 4
Build 4.1.10329.0 Released May 8, 2012
Build 4.1.10111.0 Released February 14, 2012
Build 4.0.60831.0 Released October 11, 2011
Build 4.0.60531.0 Released June 14, 2011
Build 4.0.60310.0 Released April 19, 2011
Build 4.0.60129.0 Released February 14, 2011
Build 4.0.51204.0 Released December 15, 2010
Build 4.0.50917.0 Released September 28, 2010
Build 4.0.50826.0 Released September 1, 2010
Build 4.0.50524.00 Released June 3, 2010
Build 4.0.50401.00 Released April 15, 2010

Silverlight 3
Build 3.0.50611.0 Released August 10, 2010
Build 3.0.50106.00 Released January 19, 2010
Build 3.0.40818.00 Released September 1, 2009
Build 3.0.40723.00 Released July 27, 2009
Build 3.0.40624.00 Released July 9, 2009

Silverlight 2
Build 2.0.40115.00 Released February 18, 2009
Build 2.0.31005.00 Released October 14, 2008

Silverlight 1
Build 1.0.30715.00 Released July 23, 2008
Build 1.0.30401.00 Released April 4, 2008
Build 1.0.30109.00 Released Jan. 15, 2008
Build 1.0.21115.00 Released Nov. 20, 2007
Build 1.0.20816.00 Released Sept. 4, 2007

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Full release details of Microsoft Silverlight Release History document.