Psoriasis is a chronic, inflammatory disease affecting 1-3% of the world's population. Joints can be affected in up to 30% of patients. About one third of patients have either severe or moderate (involving more than 10% of body surface area) disease. Patients affected with extensive psoriasis have an impaired quality of life. Psoriasis has a large spectrum of clinical features and evolution, so no complete agreement on the classification of the clinical variants exists. Plaque psoriasis is the commonest form (more than 80% of affected patients). The course of plaque psoriasis varies. Spontaneous resolution is possible, but rarely occurs. Plaques tend to remain static or slowly enlarge. Flexural (inverse, intertriginous) psoriasis manifests with lesions thinner than those of plaque form with no or minimal scaling, and is localized in the skin folds. Guttate (eruptive) psoriasis has frequently a sudden onset and frequently appears abruptly after a bacterial or viral febrile episode of inflammation of the upper ways. Pustular and erythrodermic psoriasis are the most severe clinical variants. In the diffuse pustular form recurrent episodes of fever occur, followed by new outbreaks of pustules. Erythrodermic psoriasis corresponds to the generalized form of the disease. The entire skin is bright red and is covered by superficial scales. Fatigue, myalgia, shortness of breath, fever and chills may also occur. In sebopsoriasis (seborrheic dermatitis + psoriasis) the lesions tend to occur at the same sites as seborrheic dermatitis; greasy scales predominate, but silvery scales can be found in some areas. Nail psoriasis shows various features: nail pits; oil spots; subungual hyperkeratosis; onycholysis. Rare forms include psoriasis circinata, lip psoriasis and oral psoriasis. Differential diagnosis includes many other dermatological conditions.