Azoospermia, defined as the absence of sperm in the ejaculate after examination of the centrifuged specimens, affects about 1% of the male population and 10-15% of infertile men. In about two-thirds of cases, this is caused by severe spermatogenic dysfunction, and it is commonly termed "nonobstructive azoospermia" (NOA) to differentiate it from the less severe form of azoospermia caused by the obstruction of the seminal tract (obstructive azoospermia-OA), the latter affecting the remaining one-third of cases. Managing patients with NOA is challenging due to the severity of spermatogenic dysfunction and the lack of medical treatments, with surgical retrieval of testicular sperm being the only way of enabling some of these patients to father their own biological children. In-depth clinical knowledge is key for supporting clinical reasoning and decision making when counselling patients with NOA, and surgical skill is required to maximize the outcome of surgical procedures that aim to retrieve testicular sperm. The present book is a collection of scientific papers published in a Special Issue of the Journal of Clinical Medicine , authored by some of the most influential researchers in the field. The Special Issue, and thus also this book, were conceived to provide early career reproductive urologists and endocrinologists with an update of the scientific evidence in the field, together with surgical tips.