In contrast to popular belief, a rich variety of morphological characteristics exists in the imperfect states of powdery mildews. Because it has been generally assumed that species cannot be distinguished by their appressoria, haustoria, conidiophores, conidia, fibrosin bodies, and conidial germ tubes, their morphology has received little attention and several older publications have even been forgotten. As with the perfect states, few species can be recognized by one characteristic of the imperfect state alone but many species can be identified when a combination of several characteristics is used. Important characteristics are the location of the mycelium, the production of conidia singly or in chains, the presence or absence of conspicuous fibrosin bodies, the appressoria, the size and shape of the conidia, and the position and type of their germ tubes. Many species are associated with particular families or genera of plants and therefore these are included in a key to identify 131 species of powdery mildew. This key shows how much and especially how little is known about many species. It is hoped that this review will stimulate study of the morphology of the imperfect states of numerous species. Consideration of both the perfect and the imperfect state should result in a more natural classification of several genera, for exampleUncinula andErysiphe which at present include both species which produce conidia in long chains and those which produce conidia singly. It appears that there are two lines of development of the imperfect states. One is characterized by lobed appressoria and conidiophores which produce conidia singly. The other is characterized by more or less rounded, unlobed appressoria and conidiophores which produce conidia in chains. A better knowledge of all the different imperfect states may provide more information regarding the evolution of powdery mildews.