There are many procedures available for evaluating semen, and except for culling obviously inferior samples, none of the procedures is highly correlated with fertility. The most commonly used method is estimating sperm motility subjectively, and this method is appropriate for culling substandard semen; this approach also can be used for more stringent quality control and for experiments if done “blindly” by well trained persons. However, computerassisted sperm analysis is more objective. Currently, the bovine AI industry frequently uses flow cytometry to measure sperm cell membrane integrity; this is an excellent procedure for evaluating sperm that is rapid, precise, objective, and reasonably correlated with fertility. In vitro fertilization would seem to be an excellent functional assay of spermatozoa, but results are not always well correlated with in vivo fertility, probably in part due to artifacts of in vitro capacitation. A rarely used method of evaluating sperm is competitive in vivo fertilization, which is very sensitive for detecting treatment or bull-to-bull differences in fertility. However, it requires genetically marking sperm and then evaluating embryos, fetuses, or offspring. The competitive fertilization approach is particularly useful for evaluating sexed semen because sex can be used as the genetic marker, treatments can be evaluated within the same male, and fetuses of cattle and horses are easily sexed non-invasively via ultrasound.