Embryo multinucleation (MNC) is a common problem in assisted reproductive technology (ART) and should serve as a morphological marker of embryo quality. There may be more than one mechanism to explain embryo MNC; some investigators believe that the pathophysiology involved in binucleation and MNC may be different. Chromosomal defects occurring during early cleavage stages of human embryo development are common and result in embryos with limited developmental capacity and potential to implant when transferred during ART procedures. A comprehensive classification of embryo MNC to help communicate embryologists and clinicians, clinicians and patients, and also as a tool for future scientific reporting on these frequent embryological findings. Embryo MNC is a common event in human clinical embryology. Binucleated embryos appear to have a better prognosis compared to micronucleated embryos in reaching blastocyst stage in vitro, or achieving a pregnancy if transferred.