Abnormalities in the luteal phase have been detected in virtually all the stimulation protocols used in in vitro fertilization, on both the hormonal and endometrial levels. Supraphysiological follicular or luteal sex steroid serum concentrations, altered estradiol: progesterone (E2/P) ratio, and disturbed luteinizing hormone pituitary secretion leading to corpus luteum insufficiency or a direct drug effect have been postulated as the main etiologic factors. Luteinizing hormone supports corpus luteum function, and low LH levels have been described after human menopausal gonadotropin treatment, after gonadotropin-releasing hormone (GnRH)-agonist treatment, or after GnRH-antagonist treatment. These low luteal LH levels may lead to an insufficient corpus luteum function and consequently to a shortened luteal phase or to the low luteal progesterone concentrations frequently described after ovulation induction. A direct effect of the GnRH agonist or GnRH antagonist on human corpus luteum or on human endometrium and thus on endometrial receptivity cannot be excluded, as GnRH receptors have been described in both compartments. Endometrial histology has revealed a wide range of abnormalities during the various stimulation protocols. In GnRH-agonist cycles, mid-luteal biopsies have revealed increased glandulo-stromal dyssynchrony and delay in endometrial development, strong positivity of endometrial glands for progesterone receptors, decreased αvβ3-integrin subunit expression, and earlier appearance of surface epithelium pinopodes. These factors suggest a shift forwards of the implantation window. Progesterone supplementation improves endometrial histology, and its necessity has been well established, at least in cycles using GnRH agonists.