Allomaternal care (AMC) (i.e., infant care that is provided by group members other than an infant's mother) is a rare, although phylogenetically widespread, mammalian infant care strategy. In primates, however, AMC occurs at unusually high frequencies, particularly among several haplorhine (monkey and ape) taxa. In fact, AMC is present in every major primate radiation and has been described in 74% of 154 species for which data are available. Its widespread presence in the Order Primates suggests that there may have been strong selective pressure for AMC early in primate evolution, but it is currently unknown why these behaviors are so common in primates. Research focused on captive callitrichids (tamarins and marmosets) has contributed greatly to our understanding of the potential causes and consequences of highly derived forms of AMC (i.e., cooperative breeding). Recent efforts have shifted focus to understand the selective pressures leading to the expansion and diversification of AMC throughout the Primate Order, thus expanding research to investigate the causes and consequences of less derived forms of AMC. Here we review the broad-scale patterns observed in primates and outline innovative and exciting avenues of research moving forward.