This book provides important research advancements on prolactin. Prolactin is a polypeptide hormone which is synthesized in and secreted from the lactotrophs of the anterior pituitary gland. However, we are now familiar with the fact that secretion and synthesis of prolactin is not limited to the anterior lobe of the pituitary gland, but other organs and individual cells can also produce it. This book provides an introduction to follow a course of integrated knowledge on this topic and its research during the past two to three decades and it may also help us understand some of the concerns that we face today.