The various species concepts with emphasis on those which can be applied to Aspergillus and its teleomorphs are discussed. Any proposed new species should show evidence for evolutionary divergence from other taxa, particularly unique DNA characters at multiple loci and the polyphasic approach was suggested as the 'gold standard' for species delimitation using a combination of multilocus sequence data, morphological, physiological characteristics and ecological data. For species descriptions it is recommended to examine several gene sequences (e.g., ITS, calmodulin, beta-tubulin, actin) and submit them to recognized sequence databases. Dual naming of Aspergillus taxa with teleomorphs has been recommended where necessary. To avoid confusion, the 'List of Names in Current Use' is recommended as a reference for Aspergillus nomenclature. For clinical researchers who depend on one of the names, it was suggested to use the name 'complex' if identification is solely based on morphology, which cannot distinguish between closely related species. A protocol for naming new Aspergillus taxa is proposed, which include the deposition of type cultures in at least two recognized culture collections. If type cultures are not available the taxon can be declared invalid.