The impact of psychosocial factors on the incidence and progression of cancer has become an area that demands attention. In this article recent evidence of psychosocial effects on cancer incidence and progression is reviewed in the context of past research. Psychosocial factors discussed include personality, depression, emotional expression, social support, and stress. Mechanisms that could mediate the relationship between psychosocial conditions and cancer incidence and progression are also reviewed. These include alterations in diet, exercise, and circadian cycles; variations in medical treatment received; and physiological mechanisms such as psychoendocrinologic and psychoneuroimmunologic effects. We conclude that there is a nonrandom relationship among various psychosocial factors and cancer incidence and progression that can only partially be explained by behavioral, structural, or biological factors. Suggestions for future research are discussed.