This is a special 50th year anniversary volume of Advances in the Study of Behavior with contributions from past and present editors and authors of the serial. Initiated 50 years ago, Advances in the Study of Behavior strives to serve the increasing number of scientists engaged in the study of animal behavior. That number is still expanding. This volume makes another important contribution to the development of the field by presenting theoretical ideas and research to those studying animal behavior and to their colleagues in neighboring fields.