Infertility is an increasing problem in many societies. The success rate of assisted reproduction is still below 30%, although the treatments and techniques have improved continuously. An important but often underestimated aspect of fertility and infertility is the immunology of gametes and the early embryo. Sperm is a potential target for autoimmunity in men and alloimmunity in women, but physiologically, such immunoreactions do not occur. Although the preimplantation embryo is semiallogeneic to the mother, no destructive reaction occurs but her uterus prepares a unique, finely balanced immunological environment around the implantation site, which is indispensable for successful early pregnancy. This book provides an overview of the current knowledge of physiological and pathological immune reactions which favor or disturb fertility. The regulation of embryo implantation and trophoblast invasion through cytokines, growth factors and other pregnancy-related immuno-regulatory molecules is described in detail.Autoimmunity and allergy are presented as possible causes of reproductive failure. With contributions by leading international groups this volume provides a valuable introduction for those readers who are not yet familiar with the immunology of reproduction while it also serves to update specialists on the most recent findings in the field.