No taxonomic concept has so far been proposed that can account for all isolates ofLeveillula: most of their morphological characteristics are too variable, and their pathogenicity too flexible to accommodate them in well-defined taxa. We therefore distinguish here only between loosely defined populations of the fungus, with various geographical and host distributions. A description is presented of the morphology of the vegetative and reproductive organs ofLeveillula, of the infection and colonization process (chiefly on pepper) and of the degrees of endophytism on various hosts. The symptoms produced by the mildew on its hosts are reviewed. Host reaction to mildew invasion differs considerably, especially because on some hosts, but not on others, extensive leaf shedding is induced. The host range ofLeveillula fungi extends over 74 families, ca. 390 genera and 1000 species. The largest number of host species is found in the Compositae (227) and Leguminosae (154), but the most important crop hosts belong to the Solanaceae. Isolates from many hosts have been proved able to cross-infect various other host species in various genera of the host’s own family and of a number of other families. The geographical distribution ofLeveillula populations centers on central and western Asia, and on the Mediterranean region, but some major hosts are also commonly attacked in subsaharan Africa, and South and Southeast Asia, less frequently in the Americas, and occasionally in Australia. It is difficult to generalize on the epidemiology ofLeveillula mildews, (a) since the infectivity and host range of isolates differ so widely, and hence sources of inoculum have to be determined in every case by local studies, and (b) because the environmental conditions conducive to infection differ greatly on various hosts. All hosts are increasingly prone to attack as they age. Many hosts are attacked chiefly or only under warm and dry conditions, but there are some important exceptions (e.g., pepper). Yield reductions are in many countries caused on pepper, tomato, eggplants, and artichokes, in tropical countries on leguminous crops in which the pods are affected; more rarely under dry conditions on cucurbits and some ornamental hosts. Large differences in susceptibility toLeveillula exist in the varieties of major crops, but no immune varieties have so far been bred. Irrigation management is an important control measure on crops attacked under dry conditions (e.g., tomatoes): overhead irrigation is then preferable to other methods. Where wet and dry seasons alternate, proper choice of sowing seasons may succeed in minimizing attack of the mildew. Proximity of infected crops to others liable to infection by the same isolate must be avoided. The principles of fungicidal control of this mildew are discussed. The distribution, specialization, and control ofLeveillula is described in detail for 13 angiosperm families, which comprise the most numerous and/or the economically most important hosts. A list is presented of records published up to 1986 of host species, arranged by host families.