The literature on paternal behavior in rodents includes reports of many species; however, paternal behavior appears more prevalent among muroid and hystricomorph rodents than in the sciurids. Many reports of paternal behavior are based on small numbers of animals observed under special conditions. Where systematic studies have been conducted, the most salient feature of rodent paternal behavior is its lability. The occurrence varies with observer constraints, genotype, experience, social conditions during observation, and stimulus characteristics. This intraspecific variability renders application of the comparative method in understanding evolution difficult but may provide a key in the remarkable adaptation of rodents to diverse habitats. Although species differences exist the evolution of rodent paternal behavior appears to entail changes in the thresholds and conditions for the occurrence of a relatively few basic motor patterns of paternal care.