The physiological functions of the two gonadotropins, follicle-stimulating hormone (FSH) and luteinizing hormone (LH), are quite well known in both males and females. Nevertheless, the recent unraveling of structures of the gonadotropin and gonadotropin receptor genes has provided us with tools to obtain completely new information on the functions of these hormones at the molecular level. Several gain- and loss-of-function mutations of these genes have been discovered in humans, and the first knock-out experiments with these genes in the mouse have been recently published. The purpose of the present review is to describe the current state of knowledge on human mutations of FSH and its receptor, as well as on relevant animal models.