Over the past three decades, laboratory and clinical research findings have shown that T cells are the primary mediators of psoriasis pathogenesis and that psoriasis can be treated by eliminating these T cells or interfering with their activation or activity. Based on these observations, many new biologic therapies to treat psoriasis are now in development. These agents, developed primarily through recombinant DNA techniques, are designed to target T cells and the immunologic cascade associated with their activation. Four basic strategic approaches that focus on the steps involved in the immunopathology of psoriasis are: 1) elimination of the pathogenic T cells; 2) inhibition of T-cell activation, proliferation, and migration; 3) immune deviation to down-regulate the type 1 (TH1) response predominant in psoriasis; and 4) blockade of cytokine production. The goal of these new therapies is to improve the treatment of psoriasis, particularly moderate to severe disease, with agents that are well tolerated and safe for long-term use.