Version history:

.Net 1.0: February 2002
.Net 1.1: April 2003
.Net 2.0: January 2006   --   Release coincided with VS2005
.Net 3.0: Novenber 2006   --   Release coincided with Vista
.Net 3.5: November 2007
.Net 3.5 SP1: August 2008
.Net 4: May 2009   --   Release coincided with Windows 7
.Net 4.5: August 20012   --   Release coincided with Windows 8
.Net 4.5.1: October 2013   --   Release coincided with VS2013
.Net 4.5.2: May 2014