The diagnosis and characterization of infertility in the male dog depends largely on the initial history and semen evaluation and on following the course of infertility over time. The diagnostic testing described here is helpful for guiding therapy, but the prognosis in serious cases of infertility is usually poor. Cases of subfertility that are not progressive have a better prognosis with optimal breeding management. Finally, the clinician should remember that some insults to the testes are reversible with time so it is important not to administer therapy that will interfere with the dog's ability to recover. Client education is an important part of the management of male dog infertility because it is not uncommon for various drugs to be administered by the breeder, or at the breeder's request, without a sufficient diagnostic work-up. In addition, errors of breeding management may play a role in male dog infertility or limit the success of treatment.