This book highlights the role of Indoleamine 2, 3-dioxygenase (IDO) in the context of bovine pregnancy establishment. It is an enzyme that catabolizes the first step of Tryptophan metabolism via the kynurenine pathway and is also the rate-limiting enzyme of tryptophan catabolism. It is one of the most important immuno-modulatory molecules that play a significant role in the modulation of the maternal immune system thereby rendering a favorable environment for the survival of the semi-allogeneic fetus. This book focuses on a different mechanism of immune modulation via IDO and its crucial role in the establishment of pregnancy.