This review examines a half century of thought about the role of psychological factors in chronic pain. Changing views are discussed, and representative examples of pain research based on psychoanalytic, behavioural, cognitive, and psychophysiological theories are presented and evaluated. The evolution of thought from linear causal models of pain to multicausal explanations provides a conceptual framework for discussion. Studies reviewed show that an earlier concept, based on simple formulations of psychological causation, has been replaced by more comprehensive explanations comprising both physical and psychological influences. Further methodological and conceptual problems are discussed in the second paper of this 2-part review.