Testicular cancer is an uncontrolled division of cells in the testicles, a pair of oval glands of the male reproductive system. The testicles, also called testes, hang behind the penis in a sack called the scrotum. Testicular cancer is a relatively rare type of cancer. But unlike many other cancers, it occurs primarily in men under 35 years of age. Research shows that the incidence of this type of cancer may be slowly rising.