Psoriasis is a common, chronic inflammatory skin disease that affects 1-3% of the world population. Four subtypes of psoriasis have been reported: plaque psoriasis (psoriasis vulgaris), eruptive psoriasis (psoriasis guttata), pustular psoriasis (psoriasis pustulosa), erythrodermic psoriasis (psoriasis erythrodermica). Plaque psoriasis is the most common form of the disease. It is characterised by well-demarcated, erythematous plaques with silvery scale. Common affected sites include the scalp, elbows, knees and lumbosacral area. Psoriasis can be provoked by non-specific triggers such as mild trauma, sunburn or chemical irritants. Systemic drugs (lithium, non-steroidal anti-inflammatory agents, antimalarials, beta blockers,angiotensin-converting enzyme inhibitors) may exacerbate the disease. Pathogenesis of psoriasis is a result of a complex interplay among genes, immune system, keratinocytes and environmental factors. Dendritic cells and T helper cells have been shown to be essential to the pathogenesis of psoriasis. Cytokines produced by these cells stimulate hyperproliferation of keratinocytes and migration of inflammatory cells into the skin. The research into the immunopathogenesis of psoriasis has resulted in several highly specific and effective therapies and has changed the way psoriasis is treated.