Hydatidiform moles (HMs) are abnormal conceptions of excessive trophoblast development resulting in abnormal human pregnancies with no embryo and cystic degeneration of the chorionic villi. Prompt diagnosis, treatment and follow-up of patients using assays for betahCG from centres that specialise in this condition enable early diagnosis of potential malignant change. Hydatidiform moles occur quite frequently and although recurrence is rare, women who have experienced one molar pregnancy should be aware that they are at an increased risk of a further molar pregnancy in comparison to other women in the general population. For some women multiple molar pregnancies occur. In these women the recurrent molar pregnancies may be non-familial, referred to as recurrent molar hydatidiform moles in this article, or may result from an inherited predisposition, which we refer to as familial recurrent hydatidiform moles. We use the term familial biparental hydatidiform moles (FBHMs) for cases in which the parental contribution to the moles has been investigated and found to be biparental. It is important to recognise, however, that in some apparently non-familial cases, the absence of female siblings, or the absence of female siblings who have tried to conceive, may not allow the inherited nature of the molar pregnancies to manifest in more than one woman and be obviously familial. This review considers our current understanding about the aetiology of HMs and explores the mechanisms of both types of recurrent hydatidiform moles. It highlights the role that genetics can play in determining the origin of multiple molar pregnancies, which should be considered essential in providing women with accurate advice about their risk of recurrence, so allowing them to make appropriate reproductive choices.