The Karoo-Namib Region, comprising the arid lands of southern Africa, supports an extraordinarily rich and compositionally unusual flora. This region includes the winter-rainfall Succulent Karoo (part of the Greater Cape Flora), the summer-rainfall Nama Karoo, and incorporates the Namib Desert. The Succulent Karoo is home to the world's largest succulent flora and is very rich in geophytes. Species-area patterns at the regional scale (101-106km2) show that the Succulent Karoo had 2.6 times as many species per unit area than the Nama Karoo. The Succulent Karoo was also richer than other winter-rainfall, semi-arid (100-400 mmyr-1) regions of the world, having, for example, nearly four times as many species than equivalent-sized North American regions. Similarly, southern African winter-rainfall desert (<100mmyr-1) regions were exceptionally species-rich: here 331 species have been recorded in an area of 1.3 km2. The Namib Desert, which spans the Succulent and Nama Karoo regions, has up to 200 times the number of species that similar-sized areas of the Saharan Desert. On the other hand, the Nama Karoo does not appear to be unusually rich at the regional scale. The Succulent Karoo represents a major extratropical centre of plant biodiversity. Reasons for its high diversity include unusually predictable seasonal rainfall and relatively mild summers. The climatic regime has selected for short-lived and drought-sensitive shrub lifestyles, largely associated with dwarf to low leaf succulents. A combination of short generation times and limited gene flow has resulted in massive diversification within certain lineages (especially the Mesembryanthema), leading to the fine-scale discrimination of habitat and geographic space. Plant-pollinator coevolution appears to have played some role in the diversification of the region's large geophyte flora. Keywords: Arid lands, diversification, global comparisons, plant diversity, regional scale, southern Africa, Succulent Karoo.