The need to distinguish ductal obstruction from ablative testicular pathology in the azoospermic patient is the primary indication for testis biopsy. Correct surgical technique and proper tissue processing are crucial to gaining the maximum amount of information from the biopsy specimen. The authors discuss the indications and techniques of testis biopsy and then review the major pathologic entities encountered clinically. Quantitative methods for describing the abnormal states of spermatogenesis are considered.