Halohydrins react with base to give epoxides. The reaction is spontaneous because the energetic cost of introducing the ring strain (13 kcal/mol) is offset by the larger bond enthalpy of the  newly introduced C-O bond (when compared to that of the cleaved C-halogen bond). 
Formation of epoxides from secondary halohydrins is predicted to occur faster than from primary halohydrins due to increased entropic effects in the secondary halohydrin, and tertiary halohydrins react (if at all) extremely slowly due to steric crowding.
Starting with propylene chlorohydrin, most of the world's supply of propylene oxide arises via this route.  
An intramolecular epoxide formation reaction is one of the key steps in the Darzens reaction.
In the Johnson–Corey–Chaykovsky reaction epoxides are generated from carbonyl groups and sulfonium ylides. In this reaction, a sulfonium is the leaving group instead of chloride.