The values of various methods used to evaluate the luteal phase, including basal body temperature, measurement of progesterone (P), endometrial biopsy, ultrasonographic measurement of endometrial thickness, and measurement of endometrial proteins, are reviewed. Luteal phase defect (LPD) is a controversial entity. The diagnosis of this condition is best based on a histological study of the endometrium. Methods to improve the accuracy of the diagnosis are discussed. LPD is more likely to be a result of an abnormal response of the endometrium to P, than to a subnormal production of P by the corpus luteum. Many methods of treatment for LPD have been proposed but none is based on a properly controlled clinical trial. Treatment designed to improve the response of the endometrium to P may be more rewarding than P supplementation.