Anthropoid monkeys are subdivided into two large groups, the New World platyrrhine monkeys (callithricids and cebids) and the Old World catarrhine monkeys (macaques, baboons, guenons, and leaf-eating monkeys). Most taxa are poorly known from a neurobiological point of view, but many species are used for laboratory studies (in particular, some macaques and baboons [Macaca and Papio], the Patas monkey [Erythrocebus], and the Central and South American marmoset [Callthrix], owl [Aotus], squirrel [Saimiri], and capuchin [Cebus] monkeys). Aging is particularly well documented among these taxa from the long-tailed and rhesus macaques and from the squirrel monkey. The neurobiological basis of declining cortical function in primate aging remains to be defined. One possibility is that the structural integrity of the neocortex is compromised by frank neuronal degeneration, synaptic loss, or other morphologic alterations. The consensus emerging from recent studies, however, is that many cortical areas, including subdivisions of the hippocampal, prefrontal, motor, and sensory cortices known to participate critically in sensory integration and memory-related processes, are relatively resistant to cell death during normal aging in monkeys. In contrast, subcortical structures are more consistently affected in a manner that correlates with the severity of cognitive deficits. Importantly, recent ultrastructural and cellular analyses have demonstrated that subtle alterations involving the neuropil as well as restricted domains of the dendritic trees are likely to contribute massively, together with molecular changes in specific neurotransmitter receptor proteins, to the cognitive and memory deficits observed in aged anthropoid monkeys.