An outline of the evolution of parental care among the vertebrates is presented, leading to a characterization of critical features of parental care among the mammals and birds. Among the lower vertebrates (fishes, amphibia) and reptiles there are a variety of patterns of parental care. These can be divided into those that involve parental behavior and those that are nonbehavioral. Among extant species, parental behavior is not the predominant form of parental care, although it is present in many species of fish, frog and reptile. Nonbehavioral patterns of parental care predominate and are equally effective as parental behavior. Parental behavior is based on reciprocal stimulus interaction (trophallaxis) between the parent and the offspring and includes behavior directed at the nest, eggs, and developing young. Among lower vertebrates and reptiles there are increases in the complexity and completeness of parental behaviors but only among the mammals and birds has parental behavior been elaborated as the only form of parental care. Critical characteristics of mammalian parental behavior are: simultaneous onset of birth, lactation, and maternal care, rapid formation of an attachment of the mother to her offspring, synchrony in the behavioral interaction between mother and young during their development until weaning, and the significance of the mother-offspring unit as the basis of social organization. Among birds there is the period of egg incubation in the nest preceding hatching of the young, in addition to the above characteristics of mammals, but the mother-offspring unit is not the basis for social organization.