Understanding the psychology of pain and the psychology of analgesic behavior can improve your practice as a pain clinician, enrich your experience of dealing day to day with suffering, and provide you with new ways of thinking about working in pain. I start with the fundamental aspects, exploring the psychological factors that influence and structure the experience of pain, introduce specific psychological models that help one understand patient behavior, and finally focus on the evidence base for psychological interventions for pain.Psychology as an academic subject was born from physiology and philosophy. It has been influenced in its short history by political developments in social science, methodological advancements in biological science, and fashionable attempts to make itself an applied science, offering opinions and expertise in subjects that range from marketing and business to individual change through psychotherapy. It is instructive to bear in mind when reading anything about psychology that as a science it (a) occupies territory not occupied by any other science; that is, the explanation, prediction and control of behavior, (b) its theories draw on a wide range of other sciences, so such explanations can sometimes be based in the behavior of molecules, and other times based in the behavior of populations, and (c) that it slips between deductive and inductive methods, often without comment. What this means is that to the nonpsychologist it can sometimes appear to be freefloating; at its worse it operates only to state the obvious, at its best it displays astonishing perspicacity, providing explanations that enable people to act.Applied to pain, what this means is that it is unhelpful to think of a unitary psychology of pain. There are at least three psychologies that are useful to consider in this chapter. The first we call "cognitive" because it relates to private mental events, experiences of thought or perception. The second we call "social" because it relates to influences on behavior that arise from our evolutionary imperative to behave collectively. The third we call clinical, because it relates to specific attempts to intervene with individuals or groups for a desired health-related outcome. I will briefly introduce what we know about the cognitive, social, and clinical psychologies of pain. Next, the evidence for psychological interventions in acute and chronic nonmalignant pain is reviewed. Finally, we focus on what we don't know that we really need to know, in order to move forwards.First, it is important to revisit briefly the definition of pain, from a psychological perspective.