Primates are used extensively in a variety of research settings. Federal regulations in the US mandate that caretakers provide for the 'psychological well-being of laboratory primates'. One of the difficulties in implementing this law has been both in the definition of psychological well-being and in the need to deal with each primate species and, in some cases, age or sex class, uniquely. Non-human primates exhibit distinct individual differences in their behavioural and physiological responses to experimental challenges and caretaking procedures. We have been investigating what factors can predict some of these individual differences, and have found that factors both intrinsic and extrinsic are significant. Extrinsic factors found to predict individual differences in response to stressors include the nature and prior experience with the challenge, the presence of familiar peers and availability of social support. Intrinsic factors include cognitive interpretations of the challenge and temperamental differences in reactivity. These studies highlight the importance of understanding the context and individual psychology of macaques in order to provide laboratory environments conducive to their welfare, and in order to understand the impact experimental and caretaking procedures are likely to have on the health and welfare of our subjects.