Luteal phase deficiency (LPD) is a clinical diagnosis associated with an abnormal luteal phase length of ≤10 days. Potential etiologies of LPD include inadequate progesterone duration, inadequate progesterone levels, or endometrial progesterone resistance. LPD has not only been described in association with medical conditions but also in fertile, normally menstruating women. Although progesterone is important for the process of implantation and early embryonic development, LPD has not been proven to be an independent entity causing infertility or recurrent pregnancy loss. Controversy exists regarding the multiple proposed measures for diagnosing LPD and, assuming it can be diagnosed accurately, whether treatment improves outcomes. This document replaces the document entitled “Current clinical irrelevance of luteal phase deficiency: a committee opinion,” last published in 2015 (Fertil Steril 2015;103:e27–e32).