Although the interval from birth to weaning is brief for dogs and cats, it is a very intensive period of adjustment to the extrauterine environment and preparation for the relatively greater independence of post-weaning. This rapid progression of events is preceded by critical developmental transitions, including organogenesis and parturition, and it is characterized by postparturient transitions that include the 3-4d neonatal period, the 21-28 maturation, and weaning itself. The purpose of this discussion is to characterize the primary events of these five transitional zones and to describe practical methods for evaluating progression through each transition, for establishing differential diagnoses, and for supportive actions and responses.