An attempt to review recent advances of any endeavor immediately imposes difficult decisions in deciding what constitutes recent. The breadth of the present topic has intensified the need for decision-making during the preparation of this article. At the onset I would hasten to point out that the subject matter chosen for consider ation is meant to be representative and it is not implied to be complete, as it is drawn from a rapidly growing literature of research reports, books, review, and symposium articles. I have attempted to avoid cataloging numerous reports under related topic areas but rather state my impressions of current status where possible. Articles providing the basis for conclusions offered are cited where appropriate. The key to understanding variability in the fungi clearly lies in their reproductive systems. Mechanisms outside the sexual cycle which contribute and provide vari ability in these organisms must be understood. The relationship of reproductive systems to life cycles, their genetic control and function, and the involvement of extrasexual mechanisms need extensive evaluation. The role of life cycles and mat ing systems in aiding or impeding genetic investigations of pathogenicity requires careful consideration. In many fungi self-fertilization is minimized or precluded by a variety of mecha nisms which insure outbreeding and genetic reassortment in the diploid and dikar yotic forms and delay the elimination of certain genes, which may reduce fitness when homozygous. In contrast, there are many fungi which utilize a variety of inbreeding systems or for which no sexual mechanism is known. The need for genetic diversity in these organisms is either left to chance through mutation, occasional out-crossing, or other nonsexual mechanisms. Classically, it is accepted that mutation, heterokaryosis, parasexuality, physiolog ical adaptation, and cytoplasmic determinants provide variation aside from a stan dard sexual cycle. Their occurrence in the fungi has been well documented in recent