This new monographic work is a synthesis of important Chironius literature and a critical evaluation of the systematics and natural history of the colubrid snake genus Chironius. It begins, in general, with a description of the material examined and methodology used to conduct the systematic programs. The presentation of a current synopsis of the genus, a character summary, and an English key to the species, is followed by lengthy accounts of each species and their subspecies. In this section, a new species and three new subspecies are presented. The account section is followed by a discussion on interspecific relationships, formation of a phylogenetic tree, Cenozoic history of Central and South America, zoogeographic considerations, and a proposed scenario of the evolution of the genus. A short discussion of intergeneric relationships is followed by a summary of taxonomic changes proposed in the monograph. This book provides the reader with an up to date account of one of the most commonly encountered colubrid snakes of Latin America. It will be a useful tool for any serious student of snake systematics in the neotropics.