▪ Abstract In ascomycetes, the single mating type locus (MAT) controls sexual development. This locus is structurally unusual because the two alternate forms (“alleles”) are completely dissimilar sequences, encoding different transcription factors, yet they occupy the same chromosomal position. Recently developed procedures allow efficient cloning of MAT genes from a wide array of filamentous ascomycetes, thereby providing MAT-based technology for application to several ongoing issues in fungal biology. This article first outlines the basic nature of MAT genes, then addresses the following topics: efficient cloning of MAT genes; the unusual molecular characteristics of these genes; phylogenetics using MAT; the issues of why some fungi are self-sterile, others self-fertile, and yet others asexual; the long-standing mystery of possible mating type switching in filamentous fungi; and finally the evolutionary origins of pathogenic capability.