The reviews in this volume deal with questions of the mechanisms of pathogenesis and with organisms that have only recently been extensively studied on the molecular level.**The introductory section presents an overview of pathogenesis, emphasizing common elements and genetic mechanisms of regulation and a review on the population genetics of bacterial pathogenesis. The second section deals with the regulation of synthesis of surface components and their role in the colonization of the host and/or of the host immune systems. The third section covers the invasion and intracellular growth of facultative and obligate intracellular parasites. The last section is devoted to studies of the role of bacterial toxic products in pathogenesis.