The majority of ectopic pregnancies can be diagnosed on transvaginal scan before treatment. Diagnosis should be based on positive visualization of an ectopic pregnancy rather the inability to visualize an intrauterine pregnancy. The majority of ectopic pregnancies will be visible on the initial transvaginal scan performed. Those who do not have their ectopic pregnancies visualized on the first examination will be initially classified as having a pregnancy of unknown location. They will then need follow-up until a diagnosis of ectopic pregnancy is confirmed. This review discusses the use of ultrasound in the diagnosis of ectopic pregnancy and the specific criteria for diagnosis of the different types of ectopic pregnancy.