Systematic research on platyrrhine cognition expanded in the second half of the twentieth century. Since then, the combined efforts of natural observations and controlled laboratory tests have produced much evidence of cognitive abilities by means of which platyrrhines solve problems both in physical and social contexts. This entry touches several research areas on platyrrhine cognition. To date, a limited number of genera have been investigated, typically those widespread in cognitive laboratories, whereas many taxa have been less - or not at all - studied.