Large aeolian bedforms (complex and compound varieties of dunes or draas) dominate over wide areas of many sand seas and are well represented in the rock record. Analyses of allometric relationships between dune height and spacing in several sand seas suggest that draas develop by vertical growth of existing dunes. In the Namib and other sand seas draa tend to occur in areas of low total and net sand transport rates. The distribution of such bedforms in a sand sea reflects long-term patterns of deposition which are related to regional-scale changes in wind regimes and sand transport rates. Superimposed bedforms are almost ubiquitous on draa and develop only after a minimum size of draa has been reached. Formation of superimposed dunes reflects spatial and temporal variations in secondary flow patterns developed on the draa. Measurements of winds and depositional patterns on complex linear dunes in the Namib Sand Sea suggest that an important mechanism for their growth is the migration of superimposed dunes from areas of high to low wind velocity on draa flanks, leading to the deposition of sediment by bedform climbing.