There are numerous probable mechanisms for the clinical occurrence of a luteal-phase deficiency. Defects may occur in either the proliferative, luteal, or luteal-rescue stage of a menstrual cycle. In each of these three domains, alterations in the trophic stimulation or the response at either the ovarian or endometrial level further subdivide the etiologies for luteal-phase deficiency. Additional development of new concepts in the areas of intraovarian signaling, the possible role of growth factors, and the measurement of newly discovered luteal products will enable us to expand our thought process. With a better understanding of the pathophysiology of luteal-phase deficiency, it is anticipated that new treatments will be devised to address precisely a given specific etiologic factor.