This chapter provides the background information of the gestational trophoblastic disease (GTD), and discusses its prevention, diagnosis, treatment, and prognosis. GTD comprises a spectrum of premalignant to malignant conditions that originate from the placenta. Histologically distinct diseases within the spectrum of GTD include complete and partial hydatidiform moles (premalignant) to invasive moles, gestational choriocarcinoma, placental site trophoblastic tumor, and epithelioid trophoblastic tumor. Effective birth control can prevent this disease. The most common symptom is abnormal vaginal bleeding. Transvaginal ultrasonography is used to establish the diagnosis of molar pregnancy in combination with abnormally high level of human chorionic gonadotropin (hCG) (>100 000 mIU/mL). Serial monitoring of hCG is used for identification of postmolar gestational trophoblastic neoplasia. Patients with nonmetastatic GTN (Stage I) or low-risk metastatic GTN (Stage II or III) can be treated with single-agent chemotherapy.