Diagnosis of ovarian masses can be difficult because many pathologic conditions can affect the ovary and have similar clinical and radiologic manifestations. Knowledge of pathologic, age-specific characteristics can help refine the differential diagnosis. Ovarian masses are nonneoplastic (ovarian functional cysts, polycystic ovary disease, and ovarian torsion) or neoplastic (surface epithelial, sex cord-stromal, germ cell, and metastatic tumors). Functional cysts, if complicated by hemorrhage, can have a confusing ultrasonographic (US) appearance. Polycystic disease and torsion are easily diagnosed with US. Benign and malignant forms of serous and mucinous surface epithelial tumors can usually be differentiated with US. Imaging features of surface epithelial tumors of low malignant potential are nonspecific, resembling those of benign serous and mucinous tumors. Mature (benign) teratomas are usually cystic, with components of fat, soft tissue, and calcium, and are sonographically distinct from immature (malignant) teratomas, which are mostly solid. Sex cord-stromal tumors occur more often in menopausal or postmenopausal women and are typically solid. Metastatic disease is less common than other ovarian tumors; however, its radiologic appearance may resemble those of other masses.