A teratoma is a rare congenital tumor derived from embryonic germ cells. These tumors are most often found in the ovaries and testes. These tumors, which can be benign or malignant, contain cells and/or differentiated tissues that should not be found in the organ they afflict. For instance, a cancerous ovarian teratoma might contain skin cells, cartilage, and lung tissue; a cystic benign ovarian teratoma (also called a dermoid cyst) is made up of epithelial tissues and often contains hair and teeth. These tumors are most often found in children and adolescents, although they may develop later in life.