Gestational trophoblastic diseases (GTD) consist of a group of neoplastic disorders arising from placental trophoblastic tissue after normal or abnormal fertilisation. The WHO classification of GTD includes hydatidiform mole, invasive mole, choriocarcinoma, placental site trophoblastic tumour, and miscellaneous and unclassified trophoblastic lesions. GTD have a varying potential for local invasion and metastases and they generally respond to chemotherapy. Broad variations in the distribution of GTD exist worldwide, with higher frequencies in some parts of Asia, the Middle East and Africa, but the extent to which they can be attributed to methodological difficulties in obtaining accurate rates is unclear. Maternal age and a history of GTD have been established as strong risk factors for hydatidiform mole and choriocarcinoma. We review published data on the worldwide distribution of GTD, original data from cancer-registry-based statistics on choriocarcinoma, and major aetiological hypotheses, including parental age, AB0 blood groups, history of GTD, reproductive factors, oral contraceptive use, and other environmental factors.