The management of patients with a poor ovarian response (POR) to ovarian stimulation represents a challenging issue in reproductive medicine. Apart from economic burdens, the patient with POR has poor prognosis in assisted reproductive technology (ART), which represents a common cause of drop-out from treatment. To introduce a more nuanced picture of POR, the POSEIDON group developed novel criteria to identify and classify patients with low prognosis who undergo ART. The primary goal of POSEIDON criteria is to offer clinicians a pragmatic system to guide therapeutic management with the mindset to obtain the number of oocytes needed for improving ART success. A novel marker of ART success, namely, the ability to obtain the number of oocytes required for achieving at least one euploid embryo for transfer in each patient, is aligned with the POSEIDON criteria. A novel prediction model (ART Calculator) is developed to help clinicians estimate the POSEIDON marker of success. Furthermore, the POSEIDON criteria can also be used to identify more homogeneous populations to test in interventional trials.