In most species, each sex produces gametes, usually either sperm or oocytes, from its germline during gametogenesis. The sperm and oocyte subsequently fuse together during fertilization to create the next generation. This review focuses on spermatogenesis and the roles of sperm during fertilization in the nematode Caenorhabditis elegans, where suitable mutants are readily obtained. So far, 186 mutants defective in the C. elegans male germline functions have been isolated, and many of these mutations are alleles for one of the ∼60 spermatogenesis-defective (spe) genes. Many cloned spe genes are expressed specifically in the male germline, where they play roles during spermatogenesis (spermatid production), spermiogenesis (spermatid activation into spermatozoa), and/or fertilization. Moreover, several spe genes are orthologs of mammalian genes, suggesting that the reproductive processes of the C. elegans and the mammalian male germlines might share common pathways at the molecular level. Developmental Dynamics 239:1502–1514, 2010. © 2010 Wiley-Liss, Inc.