This review summarises the fundamental immunologic principles regarding the cellular and molecular relations involved with immune function in a normal gestation. During the past decades, more information has been gathered to explain these complex immunological mechanisms. These mechanisms include both fetal factors such as trophoblast cell properties and altered MHC Class I expression and local maternal factors such as specialized uterine natural killer cells and a shifting of the T-helper cell cytokine profile from a type 1 to a type II array. Other novel immunomodulators are found to be expressed in the local uterine environment to aid in fetal survival. Increased knowledge about the immune relationship between the mother and the fetus permits not only a better understanding about pathological outcomes of pregnancy and gestational complications of autoimmune diseases but also the development of therapeutic approaches for solid organ transplantation.