The differentiation of non-tumorous ovarian cystic disease from an ovarian tumor (especially a malignant tumor) is important for managing patients with cystic ovarian mass, because most ovarian diseases are benign and the majority is a functional ovarian cyst. The most common ovarian cyst that comprises a non-tumorous ovarian cystic lesion is a functional cyst. Non-tumorous ovarian cystic lesions are usually incidental findings, but they sometimes cause symptoms due to mass effect, hemorrhage, or rupture. In addition, many non-tumorous ovarian cysts may raise the suspicion of malignancy. Transabdominal and transvaginal ultrasound are important not only in the detection but also in the diagnosis of ovarian cystic disease. Computed tomography and MRI are the dominant imaging modalities for the differentiation of non-tumorous ovarian cystic disease from ovarian tumors. Although imaging modality is crucial for accurate diagnosis, considering the clinical history is equally important.