The immune system has evolved mechanisms enabli ng it to discriminate between self and non-self constituents. A normally functioning organism is naturally unresponsive to self antigens originating from clonal deletion, aner­ gy, or an active suppression of self-reactive T cells. The wide spectrum of autoimmune responses may be divided into systemic and organ-specific dis­ eases and encompasses the actions of autoantibodies, autoantigeu-autoanti­ body complexes, and sensitized T lymphocytes. The aim of this book is to review the current understandin g, concepts and hypotheses regarding the pathogenesis and therapeutic approaches in autoim­ mune disorders. The first six articles are generally concerned with the mech­ anisms operating in autoimmune phenomena, whereas the subsequent reviews discuss particular phenomena in representative autoimmune dis­ eases. The introductory article underlines the significance of the interactions between antigen-specific T cells and other elements ot the immune system and proposes that not suppression, but rather activation of the immune sys­ tem, leading to a redirection of the immune response, would be therapeuti­ cally beneficial. The next articles present the importance of costim ulatory signals in determining various types of immune responses, including autoim­ munity, and the role of regulatory T cells. Further, an interesting phenome­ non of epitope spreading as a mechanism for the progression of autoimmune diseases is presented.