This volume synthesizes results of experiments using invertebrate, vertebrate, and primate model systems to explore the biological processes that mediate behavioural and affective responses to stress. Beginning with studies of invertebrates and progressing through studies of more complex organisms, the book examines cellular and neural mechanisms in fear conditioning, biological processes in the response to uncontrollable stressors, and neural systems in motivation and affect. The results presented reveal biochemical and cellular mechanisms and neural systems that have been conserved through evolution and that mediate stress reactions such as anxiety and depression in humans. The later portion of the book develops new theories of human emotion, personality and psychopathology based on our understanding of learning in animal models.