The study of parasites and their interactions with hosts continues to represent a challenging area of modern biology. The availability of new techniques and instrumentation, coupled with the development of daring new hypotheses and concepts, has paved the way for the dramatic evolution of parasitology from a static descriptive endeavor to a dynamic one based on biochemistry, immunology, molecular biology, and modern cell biology. Studies of this nature obviously fall within the domain of pathobiology. Consequently, when the contributions included in this volume of Comparative PathobioZogy were offered to this series. after critical review, we welcomed the opportunity to make them available to the scientific community. The contributions included herein represent presentations delivered before enthusiastic audiences at three different symposia, all held in 1983. The first, entitled "Some Aspects of Modern Parasitology", was organized by Dr. Gary E. Rodrick of the University of South Florida and myself on behalf of the American Society of Zoologists. The chapters by C. E. Carter and B. M. Wickwire. B. J. Bogitsh, and W. M. Kemp were originally presented at that symposium. The second symposium. organized by Dr. G. Balouet of the Faculte de Medecine, Brest, France, and myself on behalf,of the Society for Invertebrate Pathology, was entitled "Cellular Reactions in Invertebrates." The chapters by G. Balouet and M. Poder and M. Brehelin were originally presented at this symposium.