Most organisms consist of two cell lineages - somatic cells and germ cells. The former are required for the current generation, and the latter create offspring. Male and female germ cells are usually produced during spermatogenesis and oogenesis, which take place in the testis and the ovary, respectively. Spermatogenesis involves the differentiation of spermatogonial stem cells into spermatocytes via mitotic cell division and the production of haploid spermatids from the tetraploid primary spermatocytes via meiotic cell division. Spermatids subsequently give rise to spermatozoa in the final phase of spermatogenesis, called spermiogenesis. These fundamental steps, where mitotic proliferation precedes meiosis during spermatogenesis, are observed in a wide variety of organisms. However, developing a comprehensive understanding of the cell biology and genetics of spermatogenesis is difficult for most species because it occurs within a complex testicular environment characterized by the intimate association of developing sperm with accessory cells. In this Primer, we summarize the processes of spermatogenesis occurring in two pivotal model animals - mouse and Caenorhabditis elegans - and compare them to consider which important features might be evolutionarily conserved.