We introduce the thematic collection by noting some striking similarities in the cognitive abilities of human infants and nonhuman primates. What are the implications of these similarities for our comprehension of human infant cognition? After providing a brief historical and conceptual background on comparative behavioral research, we discuss how work with nonhuman primates has impacted theories of core knowledge, domain specificity, and language. We conclude by previewing the articles comprising this special issue of "Infancy." (Contains 3 footnotes.)