The increasing utilization of a gonadotropin-releasing hormone agonist ovulation trigger and the widespread use of artificial cycles for the transfer of frozen-thawed or donated embryos has renewed interest in the luteal phase in assisted reproductive technology. The "luteal phase defect" phenomenon is now understood as a continuum: there is less impairment with milder stimulation than with more intense ovarian stimulation, and less impairment after controlled ovarian stimulation and human chorionic gonadotropin ovulation triggering than after gonadotropin-releasing hormone agonist ovulation triggering, the latter being associated with rapid luteolysis. The intensity of the support of luteal phase necessary to achieve optimal outcomes therefore depends on the degree of luteal phase defect encountered in different treatment protocols. Ultimately, the luteal phase of an artificial cycle in which ovulation is suppressed, a corpus luteum is therefore absent, and the establishment of endometrial receptivity completely relies on the orchestrated exogenous administration of sex steroids, is the litmus test situation for the study of the efficacy of different luteal phase support preparations, doses, regimens, and routes of administration.