This volume provides an overview of the state of knowledge concerning this intriguing protein kinase. It brings together contributions from leading investigators engaged in research in this field. Key developments during the past three years pertain to the elaboration of the crystal structure and definition of novel functions of the kinase, such as its role as an inhibitor of apoptosis. Additionally, the shuttling of the kinase to various compartments in response to physiological and stress stimuli appears to be a key feature of the functional regulation of its activity in the cell.