Over the past two decades there has been an association between assisted reproductive technologies and increased monozygotic twinning. The association is not clear nor are the causes of assisted reproductive technology-related monozygotic twinning understood, although there are several theories as to the possible mechanisms involved. This review looks at some of the assisted reproductive technologies which may be associated with an increased risk of monozygotic twinning such as intracytoplasmic sperm injection, assisted hatching and blastocyst transfers. Determining the true incidence of monozygotic twinning after assisted reproductive technologies is important as there is a well-documented increase in perinatal morbidity and mortality of monozygotic twins compared to singleton and dizygotic pregnancies.