The neuroendocrine control of reproduction in all mammals is governed by a hypothalamic neural network of approximately 1,500 gonadotropin releasing hormone (GnRH) secreting neurons that control activity of the reproductive axis across life. Recently, the syndrome of human GnRH deficiency, either with anosmia, termed Kallmann Syndrome (KS), or with a normal sense of smell, termed normosmic Idiopathic Hypogonadotropic Hypogonadism (nIHH), have proven important disease models that have revealed much about the abnormalities that can befall the GnRH neurons as they differentiate, migrate, form networks, mature and senesce. Mutations in several genes responsible for these highly coordinated developmental processes have thus been unearthed by the study of this prismatic disease model. This paper discusses several of the more important discoveries in this rapidly evolving field and puts them into a developmental and physiologic context.