This book provides a comprehensive and up-to-date treatment of researc h carried out in the last twenty years on congruences involving the va lues of L-functions (attached to quadratic characters) at certain spec ial values. There is no other book on the market which deals with thi s subject. The book presents in a unified way congruences found by man y authors over the years, from the classical ones of Gauss and Dirichl et to the recent ones of Gras, Vehara, and others.