In this paper two philosophical issues are discussed that hold special interest for empirical researchers studying happiness. The first issue concerns the question of how the psychological notion(s) of happiness invoked in empirical research relates to those traditionally employed by philosophers. The second concerns the question of how we psychological phenomenon. With respect to the first I argue that ‘happiness’ as usedmin the philosophical literature has three importantly different senses that are often confused. Empirical research on happiness concerns only one of these senses and serious misunderstandings about the significance of empirical results can arise from such confusion. I then argue that the second question is indeed philosophical and that in order to understand the nature of (what I call) psychological happiness we need first to determine what a theory of happiness is supposed to theoretical and practical interests in the notion of happiness? I sketch an example of how such an inquiry might proceed and argue that this approach can shed more ligh on the nature and significance of happiness (and related mental states) than traditional philosophical methods.