Closed String Tachyon Condensation: An Overview
  These notes are an expanded version of a review lecture on closed string tachyon condensation at the RTN workshop in Copenhagen in September 2003. We begin with a lightning review of open string tachyon condensation, and then proceed to review recent results on localized closed string tachyon condensation, focusing on two simple systems, C/Z_n orbifolds and twisted circle compactifications. 
Matthew Headrick, Shiraz Minwalla, and Tadashi Takayanagi