The prevalence of psoriasis is relatively high in the general population, ranging between 0.6% and 4.8%, mainly as a result of chronicity and the absence of a cure. Although genetic-environmental interaction has been proposed as a model for the causation of psoriasis, the evidence for environmental factors is rather scarce. Risk factors, which have been documented in epidemiological studies include smoking, alcohol consumption, diet, infection, drugs, and stressful life events. Psoriasis affects the quality of life to substantial degree. Apart from a few cross-sectional surveys of large series of psoriatic patients, there have been no formal studies of the natural history and prognosis of established psoriasis. By imposing methodologic control and a numerate approach, epidemiology can offer a major contribution to understand psoriasis.