Diversity in sperm morphology and performance could be explained by differences in molecular and cellular mechanisms that underlie the processes of sperm formation in the testis and maturation in the epididymis. Furthermore, there are additional differences in mechanisms that prepare the sperm for fertilization, once they are transferred to the female tract. Another level of understanding of sperm diversity requires the analysis of selective forces mainly related to post-copulatory sexual selection. Sperm competition and cryptic female choice have been identified as important agents driving the evolution of sperm morphology and function. Learning about mechanisms and selective forces is crucial for the design of tests to assess sperm quality, either in a clinical context or to probe their fertilizing capacity in assisted reproduction or reproductive biotechnologies. In addition, this will be important to understand the impact on fertility of new pharmaceuticals or environmental factors.