Transvaginal sonography (TVS) is now the imaging modality of choice for the diagnosis of ectopic pregnancy with overall reported sensitivities of>90%. Specific sonographic criteria exist for the diagnosis of tubal and non-tubal pregnancies including cervical and caesarean section scar pregnancies. Diagnosis is based on the visualization of an ectopic mass rather than the inability to visualize an intra-uterine pregnancy. This chapter discusses the specific criteria used for the diagnosis of ectopic pregnancy and examines the literature assessing the accuracy of ultrasound as a diagnostic tool.