This paper critically reviews the distribution of microsporogenesis types in relation to recent concepts in monocot systematics. Two basic types of microsporogenesis are generally recognized: successive and simultaneous, although intermediates occur. These are characterized by dierences in tetrad morphology, generally tetragonal or tetrahedral, although other forms occur, particularly associated with successive division. Successive microsporogenesis is predominant in monocotyledons, although the simultaneous type characterizes the ‘lower’ Asparagales. Simultaneous microsporogenesis also occurs in Japonolirion and Petrosaaia (unplaced taxa), some Araceae, Aponogeton, Thalassia and Tofieldia (Alismatales), Dioscorea, Stenomeris and Tacca (Dioscoreales), and some Commelinanae: Arecaceae (Arecales), and Cyperaceae, Juncaceae and Thurniaceae (Poales). Simultaneous microsporogenesis is of phylogenetic significance within some of these groups, for example, Asparagales, Dioscoreales and Poales. An intermediate type is recorded in Stemonaceae (Pandanales), Commelinaceae (Commelinales) and in Eriocaulaceae and Flagellariaceae (Poales). There is little direct relationship between microsporogenesis type and pollen aperture type in monocots (except for trichotomosulcate and pantoporate apertures), although trichotomosulcate apertures in monocot pollen, and equatorial tricolpate and tricolporate apertures in eudicot pollen, are all related to simultaneous microsporogenesis. # 1999 Annals of Botany Company