The 29 spore-ball-forming smut fungus genera (of the total 58 recognized genera) are analysed, grouped, briefly characterized and illustrated. Problems within the genera, similarities, possible relationships and differences between genera are discussed. The following groups and genera are treated: genera with spores in pairs (Schizonella, Mycosyrinx, Geminago), genera with permanent spore balls composed of colourless spores and sterile cells and/or modified mycelia (Burrillia, Doassansia, Doassansiopsis, Heterodoassansia, Nannfeldtiomyces, Narasimhania, Pseudodoassansia, Tracya), genera with permanent spore balls composed of pigmented spores and sterile cells (Urocystis, Moesziomyces, Dermatosorus, Testicularia), and genera with spore balls containing only spores and lacking sterile cells between the spore balls. Within the latter group, the two sub-groups were distinguished: one with light, brownish spores (Sorosporium, Thecaphora, Glomosporium, Fulvisporium), and another one with dark, blackish spores (Tolyposporium, Tolyposporella, Clintamra, Orphanomyces). Heterotolyposporium has small, hyaline spores between the pigmented spore balls. Sporisorium and Macalpinomyces have sterile cells between the true or pseudo spore balls. Odd genera are: Ustacystis, Mundkurella and Uleiella. A key to the spore-ball-forming genera of smut fungi is given. The value of the spore balls in the taxonomy of the Ustilaginales is discussed. The arrangement of the genera in groups is based on the most important characters of the spore balls. It has a practical, didactic purpose and in many cases the groups do not reflect natural relationships.