This chapter on sun exposure and cancer risk covers the epidemiologic evidence for the association of ultraviolet (UV) radiation with cancer risk, the psychosocial and behavioral research on sun exposure and sun protection, and the mixed messages regarding the role of vitamin D in cancer risk. The chapter focuses primarily on sun exposure but also covers indoor tanning. The evidence is convincing for a causal relationship between UV radiation and skin cancer risk, but there appears to be a protective association for some internal cancers, namely colorectal, prostate, breast, and non-Hodgkin’s lymphoma. The recommendations for UV protection and predictors of sun exposure and sun protection behaviors are described. A summary of psychosocial and behavioral skin cancer prevention interventions and approaches for different target groups is presented, and linked to theoretical models that help to explain behavior change.