The Mycosphaerella complex accommodates thousands of taxa. Many of these species are economically important plant pathogens, notably on native and commercially propagated Eucalyptus species where they cause a wide range of disease symptoms including leaf spot, leaf blotch, shoot blight and stem cankers. Some of these diseases represent major impediments to sustainable Eucalyptus forestry in several countries where infection by Mycosphaerella and Teratosphaeria species can result in reduction of wood volume and in severe cases tree death. Extensive research has been conducted on these disease complexes over the past 40 years. The incorporation of DNA-based molecular techniques has made it possible to define and to better understand the differences between the Mycosphaerella and Teratosphaeria species occurring on Eucalyptus. These studies have also enabled refinement of anamorph and teleomorph generic concepts for the genera and thus facilitated the more accurate identification of species. They have also promoted a more lucid understanding of the biology, life cycles, population biology and epidemiology of the most important pathogens in the group.