Dogs play an important role in toxicology because of their importance as a large animal, pre-clinical model for evaluating potential toxicity in human drug development including the effects of investigational drugs on the immune system. The purpose of this paper is to review the development of the canine immune system during the fetal, neonatal and postnatal periods and to compare it with that of the human immune system. Unlike rodents, the development of the canine immune system shares many similarities to that of the human. In both dogs and humans, the immune system, including the mucosal immune system, is fully developed before birth although the maturity of the immune response may continue into the postnatal period.