Psoriasis and Psoriatic Arthritis (PsA) are chronic inflammatory diseases that have a major impact on health. The prevalence and incidence estimates of these two closely related diseases show ethnic and geographic variations, being generally more common in the colder north than in the tropics. In Europe the prevalence of psoriasis varies anywhere from 0.6 to 6.5%. In the USA, the estimated prevalence of diagnosed psoriasis is 3.15%. The prevalence in Africa varies depending on geographic location, being lowest in West Africa. Psoriasis is less prevalent in China and Japan than in Europe, and is entirely absent in natives of the Andean region of South America. There are fewer reports on the incidence of psoriasis, but a recent study from Rochester, USA showed an increasing trend over the last 2 decades. The prevalence of PsA also shows similar variation, being highest in people of European descent and lowest in the Japanese. Although, study methodology and case definition may explain some of the variations, genetic and environmental factors are important. Genetic epidemiologic studies have shown that both diseases have a strong genetic component. The strongest association is with HLA-Cw*06. Associations with a number of genes including IL12B and IL23R have recently been confirmed. Environmental risk factors including streptococcal pharyngitis, stressful life events, low humidity, drugs, HIV infection, trauma, smoking and obesity have been associated with psoriasis and PsA. Here we have reviewed the current literature on the epidemiology and genetics of psoriasis and PsA.