The BCL-2 family proteins are crucial regulators of the apoptotic process. This book covers recent structure-based insights into the mechanisms of action of antiapoptotic BCL-2 family proteins as well as into the interactions between antiapoptotic proteins and their ligands. While the main focus is on the apoptotic processes in mammals, the great part of the book is devoted to viral homologues of Bcl-2 family antiapoptotic proteins.