The extraordinary plant diversity and floristic peculiarity of southern Africa's arid lands are often overshadowed by the better studied and more widely publicized Cape Floristic Region. While Cape fynbos has its analogue in the Kwongan of Mediterranean south-western Australia (Cowling et al., 1996), the flora of southern Africa's winterrainfall karoo, with its vast number of succulent species, is in a league of its own. A major objective of this chapter is to describe this diversity and seek explanations for its origins. We do this by analysing patterns and correlates of restricted range size (or endemism), and patterns and determinants of diversity at different spatial scales. Plant biogeography, endemism and diversity. Keywords: endemism, local diversity, differentiation diversity, environmental influences, seasonal movements.