This work is a brief review of the pre- and postnatal development in the North American opossum (Didelphis virginiana) presented in two volumes. Volume I summarizes gametes and fertilization, blastocyst formationan and early organogenesis, fetal membranes and placentation, parturition and migration to the pouch, general postnatal growth and development, and histogenesis/organogenesis of the integument, musculoskeletal system, cardiovascular system, blood and blood forming organs, lymphatic organs, nervous system, eye, and ear. Volume II summarizes the histogenesis/organogenesis of the respiratory, digestive, urinary, male and female reproductive and classical endocrine systems.