Teratoma is a type of ovarian germ cell tumor whose etiology is not well understood and is currently speculated to be possibly related to factors such as abnormal differentiation of germ cells during the embryonic period. Teratoma is a very common type of ovarian germ cell tumor, mainly derived from germ cells, and can be subdivided into two types: mature teratoma (i.e., benign teratoma) and immature teratoma (i.e., malignant teratoma). Teratomas contain three tissue components in the human body: ectodermal, mesodermal, and endodermal. Therefore, there will be ectodermal tissues such as hair, oil, skin, teeth, and bone fragments in teratomas, and there may also be mesodermal or endodermal tissues such as muscle, gastrointestinal, and other tissues. Most mature teratomas arise from a single germ cell that has failed to divide at second maturity or from the intranuclear replication of a mature egg. Some immature teratomas can also occur from the fusion of two eggs, with rare karyotypic abnormalities, and abnormal karyotypes are most common in higher grade immature teratomas. Teratomas tend to occur in young women in their 20s and 30s and grow with age, with the ovaries being the most common site of occurrence. Once the diagnosis of mature cystic teratoma of the ovary is established, surgery is most often required, either immediately if it is larger than 5 cm, or at a later date if it is smaller in size and requires regular observation.