This two-volume handbook presents the different aspects of comparative psychology--behavior, cognition, learning, and neurophysiology--in a balanced and exhaustive manner. Comparative psychology is the scientific study of animal cognition and behavior from an evolutionary perspective. There are 80 chapters across the set, divided into nine parts. History and Methods constitute the first two parts of the handbook. Key events and basic questions (and controversies) that have shaped the field as well as the methods used to make those questions empirically tractable are presented here. The next three parts--Adaptation/Evolution, Genes/Hormones, and Neural Substrate--present the conceptual foundations for understanding the genesis of behavior and cognition, both from a phylogenetic and ontogenetic perspective. Finally, the next four parts (Behavior, Perception/Attention, Learning/Motivation, and Cognition/Emotion) are devoted to the core research in comparative psychology today.