This paper analyses different controversial aspects of luteal phase deficiency (LPD) on the basis of our own data and current clinical experience. Conceptual and diagnostic concerns include: (i) the lack of predictive value of mid-luteal plasma progesterone determinations regarding the progestational transformation of the endometrium, and (ii) the need for a minimum of two, or even three, endometrial biopsies for diagnosis of LPD. From an aetiologic point of view follicular phase determinants of LPD are emphasized and conflicting data on claimed predisposing factors of LPD are analysed. Primary therapeutic approaches to LPD and the effectiveness and significance of such therapy on fertility are discussed.