Historically plant pathogenic species of Mycosphaerella have been regarded as host-specific, though this hypothesys has proven difficult to test largely due to the inavailability of fungal cultures. During the course of the past 20 years a concerted effort has been made to collect these fungi, and devise methods to cultivate them. Based on subsequent DNA sequence analyses the majority of these species were revealed to be host-specific, though some were not, suggesting that no general rule can be applied. Furthermore, analysis of recent molecular data revealed Mycosphaerella to be poly- and paraphyletic. Teleomorph morphology was shown to be too narrowly defined in some cases, and again too widely in others. Mycosphaerella and Teratosphaeria as presently circumscribed represent numerous different genera, many of which can be recognised based on the morphology of their 30 odd associated anamorph genera. Although Mycosphaerella is generally accepted to represent one of the largest genera of ascomycetous fungi, these data suggest that this is incorrect, and that Mycosphaerella should be restricted to taxa linked to Ramularia anamorphs. Furthermore, other anamorph form genera with Mycosphaerella-like teleomorphs appear to represent genera in their own right.