Understanding the main determinants of male fertility would allow us to advance our knowledge of male reproductive function and lifetime reproductive strategies, and also to design appropriate tests to evaluate males and semen samples collected from them. Sperm evaluation has moved away from subjective analysis to more objective means of assessing cell morphology, dimensions, functions, and the underlying mechanisms that ultimately lead to successful fertilization and generation of viable offspring. Future studies dealing with sperm function as well as biomarker identification and the development of molecular tools will undoubtedly yield better and more reliable assays. Spermatozoa experience many complex processes and these influence, to varying degrees, the fertilizing capacity of the sperm cell. After differentiation in the testis, spermatozoa undergo several key events towards fertilization; events, which have, to a greater or lesser extent, been identified as potential indicators of sperm quality. The analysis of male reproductive function needs to consider species and individual variation, and recognize that ejaculate components, including sperm form and function, may vary between ejaculates from the same animal. It is also important to bear in mind that there are important differences between the species in terms of reproductive strategies. Species or populations may differ depending on the selection pressures that have acted on them. In the case of wild populations, selective pressures will differ markedly depending on mating systems and, in particular, the intensity of sperm competition (i.e. competition between rival ejaculates when females mate with more than one male). In the case of domestic species subjected to artificial selection, breeding schemes differ in the traits that they target (e.g. selection for milk yield, meat production, or male fertility), and these differences may have a profound influence on sperm traits. In other species (e.g. man, horses, or companion animals) many studies focus on sub-fertile or infertile patients, and this may generate a bias when trying to understand the potential relationship between semenparameters and sperm fertility. The identification of sperm parameters important for fertility also requires advances on both the development of laboratory tests for sperm assessment and the