Trophoblast cells coordinate the activities of maternal and embryonic tissues by secreting hormones, cytokines, and various growth factors that selectively and specifically gain access to maternal and embryonic compartments. Abnormalities associated with trophoblast cell growth, differentiation, or function result in impaired embryonic development. Understanding the complexities of the trophoblast cell signaling system was the focus of the Serono Symposia, USA conference entitled Tropho blast Cells: Pathways for Maternal-Embryonic Communication, held August 6-9, 1992, in Las Vegas, Nevada. The conference was designed to provide a forum for morphologists, cell biologists, endocrinologists, and molecular biologists and for scientists investigating primate, ru minant, and rodent trophoblast biology. An important outcome of the conference was the communication achieved between basic scientists and clinicians. This volume represents the contributions of the invited symposium speakers. The opening keynote address of the conference was entitled "Chorio carcinoma and the Embryo" and was presented by G. Barry Pierce, M. D., Centennial Distinguished Research Professor of the University of Colorado School of Medicine. The address is not represented in this volume, but deserves a special comment. Dr. Pierce's scientific con tributions have significantly influenced our understanding of trophoblast cells. Dr. Pierce, together with Dr. A. Rees Midgley, identified the origin of syncytial trophoblast cells of the primate placenta and discovered that these cells were responsible for the production of chorionic gonadotropin (J Exp Med 1962;115:289-94; Am J Pathol 1963;43:929-43; and Science 1963;141:349-50)."