Placentitis, an inflammation of the placenta usually caused by an infectious agent, has emerged as a leading cause of reproductive loss in the equine breeding industry. This has a large economic and emotional impact when the pregnancy progresses until close to term but unfortunately ends in abortion or the birth of a small ill-thrifty foal. The placenta is composed of the amnion, which surrounds the fetus and the chorioallantois that attach to the endometrium of the mare. These structures protect the fetus and provide gas and nutrient exchange allowing the foal to grow. When placentitis occurs it usually affects the chorioallantois compromising the foal because there is a loss of attachment of the placenta to the endometrium or the presence of infection and inflammation. In this article, Dr. Karen Wolfsdorf of the Hagyard Equine Medical Institute, discusses the causes, diagnosis and treatment of placentitis in the mare.