According to population-based European studies, 2-3% of the population are affected by psoriasis. However, commonly accepted and validated diagnostic criteria are lacking. Psoriasis patients, compared to those with other dermatological disorders, are among those who suffer the highest impact on their quality of life, and this affection is comparable to other common diseases. Several lifestyle factors including alcohol and smoking have been associated with psoriasis. There is also evidence for associations with other diseases including cancer. Treatment of psoriasis engenders considerable costs, which should always be taken into account together with the relevant clinical outcome parameters. Prevention of psoriasis is crucial, as in the case of other common chronic diseases where causative treatments are lacking, but has barely been studied.