This chapter is about the development of affect in young rhesus monkeys (Macaca mulatta) and other nonhuman primates. Researchers who study such development in any species of nonhuman primate must inevitably face many of the same basic conceptual and methodological problems encountered by those who study affect in preverbal human infants and young children. First, there is the problem of classification. While investigators dating back to Darwin (1872) have readily attributed the capability for expression of complex, human-like emotions to numerous animal species, especially nonhuman primates, the development of objective criteria for classifying and differentiating among specific affective states in these subjects has not been easy. Then there is the problem of measurement. Rhesus monkeys, like preverbal human children, cannot fill out questionnaires or answer inquiries about how they are feeling at any given time and situation. Instead, investigators must rely on measures of their subjects' behaviors for much of their information about their subjects' affect-and most knowledgable primatologists recognize that some of their subjects more readily suppress outward expressions of emotion than do others, thereby complicating the task of affective measurement. Finally, there are some basic conceptual problems of definition and differentiation. It is difficult (and perhaps meaningless) to separate emotional expression from reflexive response or from biorhythmic state in neonatal primates, and it is equally difficult to distinguish between affective and cognitive activity when neonates grow older. Clearly, the scientific study of the development of affect in young primates, human or nonhuman, is no simple or straightforward task.