A preliminary account is given of the main plant communities of the Eastern and Western deserts of Egypt, chiefly in the neighourhood of Cairo. The communities are closely related to the particular geological formations and topography of the area. The presence or absence of blown sand is very important in determining the type of community that develops. Only in areas where sand occurs (either as dunes, drift, or the sandy matrix of flinty gravel hills) is vegetation able to develop on the slopes below 700 m.; elsewhere it is confined to grooves, valleys, or depressions where soil moisture is sufficient to support vegetation. The Eastern Desert has a much richer flora than the Western Desert, due to its greater relief, heavier rainfall, and the larger variety of suitable habitats. The richest communities are those of the first terrace in deep limestone valleys, and those of hills covered with flinty gravel embedded in a sandy matrix. Most of the desert associations can be classed as edaphic or biotic climax communites. The floristic relationship between the two deserts is much closer in the psammophyte communities[long dash]especiallx_of superficial sand-drifts[long dash]than in any other type of plant community. The Nile Valley has proved a moderately effective barrier to the east-west dispersal of desert plants.