Sensitization is a concept of learning and memory that has grown out of experiments on "simple" animals. Interest in sensitization has grown tremendously in the last several years, fueled mainly by evidence of the molecular basis of sensitization in invertebrates on the one hand and the study of cocaine abuse, which produces behavioral sensitization, on the other. Because the rapid advance of information across such a broad range of research areas has made an integrated approach necessary, this volume combines findings on sensitization across the phylogenetic scale.