We offer a synthesis of the ecology and biogeography of succulent plant growth forms in arid environments. Distinctions among plants with succulent above-ground organs are discussed. Water storage is not the only possible utility of fleshy organs; dilution of salt and malate may also be important functions. In formulating a general hypothesis about conditions favoring cactoid dominance, we review Crassulacean Acid Metabolism (CAM). Regions with high levels of succulent plant diversity and dominance are enumerated together with their climates. Emphasizing potentially limiting conditions, we discuss general features of succulent CAM plant ecology. These attributes are used to derive a set of climatic characteristics favoring cactoids. Two regions possess appropriate climates: coastal deserts cooled by polar currents and semiarid subtropical montane areas. We propose that these two types of environments have been centers of succulent CAM evolution. Within these regions rocky slopes provide the most favorable habitats for CAM succulents. Most continents fit this pattern well, but Australia is an exception. The absence of native cactoids from Australia may be due to the prevalence of unfavorable climates and habitats during much of the Quaternary, but historical accidents are an equally likely explanation for this apparently empty niche. We present predictions about Tertiary and Quaternary patterns of cactoid distributions, emphasizing the importance of topographies that permit latitudinal movement.