Prepared as a tribute to Donald A. Riley, the essays that appear here are representative of a research area that has loosely been classified as animal cognition — a categorization that reflects a functionalist philosophy that was prevalent in Riley's laboratory and that many of his students absorbed. According to this philosophy, it is acceptable to hypothesize that an animal might engage in complex processing of information, as long as one can operationalize evidence for such a process and the hypothesis can be presented in the context of testable predictions that can differentiate it from other mechanisms. The contributions to this volume represent the three most important areas of research in animal cognition — stimulus representation, memory processes, and perceptual processes — although current research has considerably blurred these distinctions.