An account is given of the distribution of the sand deserts in Iran, Afghanistan and Pakistan Baluchistan, where they cover large surfaces on the plains at low and medium altitudes. Climatologically they differ considerably from each other with respect to winter temperatures, but the latitudinal sequence is obscured by elevation effects. About one half to one third of the species are strict psammophytes; structurally they are the most important components of the different plant communities and in mobile sands usually no other plants occur. Their phytogeographical and taxonomic relationships are discussed in detail and attempts made to relate the distributional patterns to certain ecological factors. 66% of the species are truly Irano-Turanian, 23% are endemics, mostly derivatives of Irano-Turanian species, 10% are Saharo-Arabian and 7% are biregionals. Further aspects pointed out are the representations of life forms of families and genera. The results in the deserts studied give support to the concept of a coherent Irano-Turanian floristic region. The southern deserts form a sub-unit of their own, characterised by high numbers of endemics, some intruding Saharo-Arabian species and an absence of several otherwise common Irano-Turanian species.