Psoriasis is a common, persistent, relapsing inflammatory skin disease that can be associated with significant morbidity. Quality of life studies in psoriasis reveal a negative impact on patients comparable with that seen in cancer, arthritis and heart disease. Patients with severe disease constitute approximately 20-30% of all patients with psoriasis, often require systemic treatment, and represent a major economic burden to the Health Service. All standard systemic therapies for severe disease are associated with the potential for major long-term toxicity, many are expensive, and a proportion of patients has treatment-resistant disease. Biological therapies or 'biologics' describe agents designed to block specific molecular steps important in the pathogenesis of psoriasis and have emerged over the last 3-5 years as potentially valuable alternative therapeutic options. Currently, biological therapies for psoriasis comprise two main groups: (i) agents targeting the cytokine tumour necrosis factor (TNF)-α (e.g. etanercept, infliximab, adalimumab) and (ii) agents targeting T cells or antigen-presenting cells (e.g. efalizumab, alefacept). Two of these, etanercept (Enbrel®) and efalizumab (Raptiva®) were licensed in 2004 in the U.K. for patients with moderate to severe psoriasis. © 2005 British Association of Dermatologists.