In 1996, The Leydig Cell, edited by Anita Payne, Matthew Hardy, and Lonnie Russell, was published by Cache River Press. This was followed by the 2007 publication by Humana Press of The Leydig Cell in Health and Disease, edited by Anita Payne and Matthew Hardy. A great deal has happened since the publication of these books, including advances in our understanding of Leydig cell formation and function and the clinical application of the basic science advancements. The new information and excitement about recent developments in the field persuaded Vassilios Papadopoulos, Dolores Lamb, Ilpo Huhtaniemi, and Barry Zirkin to edit a new book, Leydig Cells—Formation, Regulation and Function in Health and Disease. The book summarizes the current understanding of fetal Leydig cells, adult Leydig cell formation, hormonal and paracrine regulation of adult Leydig cell function, Leydig cell aging, the causes, consequences, and treatment of reductions in testosterone, and how new technologies might be used for both greater understanding of Leydig cell function and treatments related to aging- and/or lifestyle-related alterations in Leydig cell function. The chapters are written by experts worldwide. Each chapter was written independently, so there is some repetition in different chapters. There are also significant differences of opinion that we hope the readers will find interesting, provocative, and valuable.