Our understanding of the biochemistry of human sperm and fertilization has greatly expanded in the past 15 years, and continues to grow at an astounding pace. This wealth of new knowledge, elegantly organized in the first review of this series (De Jonge, 1999), brought a concomitant increase in the number of laboratory tests for human sperm function, that is, tests of the ability of sperm to capacitate and fertilize. For the first time, objective data were available for important measures such as the kinematics of capacitating sperm, the proportion of hyperactivating sperm, the ability of sperm to bind the zona pellucida, the proportion of sperm able to acrosome react, the amount of acrosin released, the ability of sperm to fuse with the oocyte and decondense, and the ability of sperm to respond to natural and pharmacologic agents involved in the pathways leading to fertilization. This new information can, and has, helped diagnose new forms of male subfertility, predict success of attempts at natural or assisted conception, and to design in vitro sperm treatments to overcome the diagnosed dysfunctions. Because of the belief among many practitioners that sperm function testing is now irrelevant due to advances in in vitro fertilization (IVF) technology, it is possible that this knowledge will no longer contribute to infertility medicine. There is no doubt that intracytoplasmic sperm injection (ICSI) has given hope to couples who previously had virtually no chance of conception with their own gametes. However, continued study of the genetic, financial, and other implications of this method is essential (Lamb, 1999; Schlegel, 1999). Until complete assurance of safety is obtained, it seems prudent and reasonable that ICSI be used only in those cases where it is absolutely needed. Unfortunately, this is not current practice; some