This is the 20th annual volume of "Progress in Heterocyclic Chemistry", which covers the literature published during 2007. As with previous volumes in the series, Volume 20 will enable the reader to keep abreast of developments in heterocyclic chemistry in an effortless way. It is a critical review of the heterocyclic literature published during 2006. It presents specialized reviews, and includes chapters, all written by leading researchers in their field.