Over the last 40-50 years studies involving thousands of testicular biopsies in boys with cryptorchidism have contributed to our knowledge of testicular histopathology and our understanding of the effects of cryptorchidism on the normal development of the germinal epithelium. Growth and maturation of germ cells and Leydig cells are crucial to allow boys to reach normal fertility potential. The following aberrations in testicular development are seen in cryptorchid testes: a decrease in total germ cell numbers, failure of fetal gonocytes (stem cells) to transform into adult dark (Ad) spermatogonia, failure for Ad spermatogonia to mature into primary spermatocytes, Leydig cell hypoplasia, and testicular fibrosis. All of these findings have been found to have a strong negative correlation with a boy's age at the time of orchidopexy. Some of these findings have prognostic significance in regards to fertility potential especially when coupled with key clinical findings such as hormonal findings, age at orchidopexy, testicle size, laterality and location of cryptorchid testes. This review focuses on key lessons learned from testicular histology in cryptorchid testes.