This book is concerned with brain function and behavior and specifically with the neural substrates of thermoregulatory, feeding, sexual and aggressive behaviors. These and other behaviors shown in Figure 1.1 are usually designated as motivated behaviors, because they are goal-directed and persistent: a tiger hunts for food; a zebra searches for water; a chimpanzee builds a nest and goes to sleep; a gull drives other birds from its home territory; a child plays with a toy; a mathematician solves a complex problem. Motivation and motivated behaviors are not very satisfactory as scientific terms, however, because they have not been defined precisely, and there have been vigorous controversies about their meaning. 1 These terms are used in this book with some reservation and are used only in a descriptive sense and as a shorthand to facilitate communication. When the neural substrates of these behaviors are better understood, these terms and such inferred concepts as drive will be unnecessary. In the meantime motivation and related terms must be used with caution, the main justification being their role as a reminder to the neurobiologist of the nature and dimension of the task of providing an experimental and theoretical analysis of behavior.