When Arthur Walton wrote these words, which appeared in the third edition of Marshall’s Physiology of Reproduction, he was unable to account for the evolutionary basis of phallic diversity in mammals, some examples of which are shown in Figure 5.1. Indeed, prior to the application of sexual selection theory to this problem (Eberhard, 1985, 1996; Parker, 1970), the presence of so much variability among male invertebrates, and among the vertebrates as a whole, had remained a mystery. The purpose of this chapter is to explore the various reasons for phallic morphological diversity, including the role that copulatory and post-copulatory sexual selection may have played in this context.