Abnormalities in sperm production or function, alone or in combination with other factors, account for 35%-50% of all cases of infertility. Although a battery of tests and treatments have been described and continue to be used in the evaluation of female infertility, the male has been essentially neglected. It would appear that the majority of programs offering advanced assisted reproduction technologies (ARTs) employ only a cursory evaluation of the male—rarely extending beyond semen analysis and antisperm antibody detection. Several factors certainly account for this disparity. First, most practitioners of ARTs are gynecologists or gynecologic subspecialists who have little formal training in the evaluation of the infertile or subfertile men. Second, the urologists, who perhaps theoretically should have taken the lead in this area, have devoted little of their literature or research budgets to the evaluation of the infertile male. Third, and perhaps most important, is the inescapable fact that sperm function testing remains a very controversial area of research. Many tests have been described, yet few have been extensively evaluated in a proper scientific manner. Those that have continue to be criticized for poor sensitivity or specificity, a lack of standardization of methodology, suboptimal study design, problems with outcome assessment, and the lack of long-term follow- up. Although many of these same criticisms could also be leveled against most diagnostic algorithms for female infertility, in that arena, the tests continue to prevail over their critics. Fourth, like female infertility, male infertility is certainly multifactorial. It is improbable that one sperm function test will prove to be a panacea, owing to the multiple steps involved in fertilization. In addition to arriving at the site of fertilization, sperm must undergo capacitation and the acrosome must allow for the penetration of the cumulus cells and the zona pellucida so the sperm head can fuse with the oolemma. In addition, the sperm must activate the oocyte, undergo nuclear decondensation, form the male pronucleus, and then fuse with the female pronucleus. Finally, with the advent and the rapid continued development of micro-assisted fertilization, sperm function testing has assumed a role of even less importance. As fertilization and pregnancy rates improve with procedures such as intracytoplasmic sperm injection (ICSI), more and more logical questions are being asked about the proper role for sperm function testing. This chapter reviews the most commonly employed techniques for sperm evaluation and examines the issues surrounding their utilization in the modern ART program.