The contributions collected in this volume represent the most recent findings, reviewed by leading scientists in the field, on several different aspects of IL-12 biology. The opening chapter analyzes the molecular regulation of IL-12 production and its immunomodulatory activities. This is followed by three contributions describing the structure of the IL-12 receptor and its role in determining the early events in the differentiation of T cell subsets. The modulation of IL-12 activities by other cytokines such as TGF-Beta, IFN-Gamma and IL-4 is also addressed. IL-12 is a key cytokine that controls infectious diseases and its role in leishmania, candida, and protozoal infections is specifically examined. In addition, IL-12 provides a new tool to explore the regulatory role of the immune system in the control of tumor development and a potential new drug which is already being tested in clinical trials. Finally, the role of IL-12 in Th1-mediated autoimmune diseases and the potential for immunotherapy based on IL-12 targeting is discussed. Collectively, these contributions give a detailed account of the present knowledge on critical aspects of IL-12. The well-edited and stimulating material offered in this volume will be of interest to all immunologists, in particular to those interested in the different facets of the Th1/Th2 dichotomy.