Vulgar psoriasis is the most common type of psoriasis, chronic skin disease which affects 1-3% of the world population. Psoriasis significantly affects the quality of life of a patient. Besides local preparations, in treatment of psoriasis phototherapy and systemic therapy are used. In treatment of moderate-to-severe psoriasis, conventional systemic therapy and biologic therapy are used today. Treatment always starts with conventional systemic therapy which, according to current guidelines for the treatment of psoriasis, includes photochemotherapy, retinoids, methotrexate and cyclosporine. In patients that do not achieve satisfying therapeutic effect with conventional systemic treatment, or they have a contraindication, biological therapy may be used. Biological agents are drugs synthesized by recombinant DNA technology and they act on specific molecular targets. Biological agents are relatively new drugs, with 10-year experience of use. Understanding the mechanisms of action of biological agents and their adverse events is very important for medical doctors and because of that screening of the patients that are considered to undergo biological therapy must be done with caution and in accordance with the guidelines. At the time of writing this thesis, two categories of biological agents are registered for the treatment of vulgar psoriasis in Croatia - TNF-α antagonists (adalimumab, infliximab and etanercept) and IL12/23 antagonists (ustekinumab). When these are used, regular supervision of dermatologist, laboratory tests and assessment of level of possible skin improvements by PASI index and Quality of Life index must be done. Numerous trials in progress are studying the treatment of vulgar psoriasis by biological agents.