Creation of mutants that affect spermatogenesis is very challenging in most experimental systems, especially mammals. The main reason this is true is because "absence of successful mating" is a negative result that can occur for a wide variety of trivial, irrelevant reasons. The C. elegans hermaphroditic mode of reproduction has unusual features that facilitate analysis of spermatogenesis. Normally, hermaphrodites are virtually 100% self-fertile and spermatogenesis defective mutants are self-sterile. A candidate spermatogenesis defective mutant will produce cross-progeny after mating to a wild type male, showing that the presence of sperm is both necessary and sufficient to restore fertility to the sterile hermaphrodite. This has allowed selection of a large number of spermatogenesis defective mutants. In this article, I will review spermatogenesis, how mutants are made and what has been learned about the identified genes and their roles during development and fertilization.