Spermatogenesis is a cyclic process during which, within each epithelial area, various generations of germ cells undergo a series of developmental steps according to a fixed time schedule. The cycle of the seminiferous epithelium can be subdivided into stages. In the mouse, 12 such stages have been described that can be distinguished from one another by steps in spermatid development. The best way to recognize the stages in seminiferous tubule cross-sections is to use Bouin's-fixed testes of normal mice and sections stained with the Periodic acid Schiff (PAS) technique and hematoxylin. Unfortunately, this is not always possible. Sometimes PAS staining cannot be used, such as when immunohistochemistry is carried out. Moreover, not all germ cell types may be present in some instances, as in young or mutant mice. We summarize here all stage-identifying criteria that can be used in the ideal situation as well as in hematoxylin-only stained sections and/or when germ cell types are missing.