This chapter provides an overview of the morphology of Aspergillus fumigatus and related species and discusses the reproductive mode of this important fungus. Recently, strains of A. fumigatus sensu lato which were previously identified as A. fumigatus in the CBS fungal culture collection based on morphology have been reexamined based on β-tubulin sequence data and randomly amplified polymorphic DNA PCR. A. fumigatus, as with the majority of species of Aspergillus, is only known to reproduce by asexual means. However, there is accumulating evidence that gene flow through recombination has occurred, or is occurring, in natural populations of A. fumigatus. Results of the genome annotation and analysis were reported in a study in which a special area of interest concerned possible reasons for asexuality in A. fumigatus. It was predicted that the genome analysis might reveal possible mutations within coding regions for genes required for sexual development, which might explain the absence of sexual reproduction in this species due to lack of an essential gene. Mating-type (MAT) genes are required for sexual development in both heterothallic and homothallic species and have been shown to be functional in A. nidulans. In the case of heterothallic species, in which complementary MAT1-1 and MAT1-2 isolates are required for sexual reproduction, MAT genes are located at a single MAT locus within the genome. In terms of its global distribution and prevalence, A. fumigatus is without doubt an evolutionarily successful organism.