The study of birth defects has assumed an importance even greater now than in the past because mortality rates attributed to congenital anomalies have declined far less than those for other causes of death, such as infectious and nutritional diseases. It is estimated that as many as 50% of all pregnancies terminate as miscarriages. In the majority of cases this is the result of faulty development. Major congenital malformations are found in at least 2% of all liveborn infants, and 22% of all stillbirths and infant deaths are associated with severe congenital anomalies. Teratological studies of an experimental nature are neither ethical nor justifiable in humans. Numerous investigations have been carried out in laboratory animals and other experimental models in order to improve our understanding of abnormal intra-uterine development. In less than two decades the field of experimental teratology has advanced phenomenally. As a result of the wide range of information that is now accumulating, it has become possible to obtain an insight into the causes, mechanisms and prevention of birth defects. However, considerable work will be needed before these problems can be resolved. This book brings together some of the more recent and important research findings related to the mechanisms and pathogenesis of abnormal develop ment. It is not only a documentation of the latest experimental work, but it also points out future directions that seem productive and challenging.