Abstract Diamond mega-placers, defined as ≥ 50 million carats at ≥ 95% gem quality, are known only from along the coast of southwestern Africa, fringing the Kaapvaal craton, where two are recognized. One is associated with the Orange-Vaal dispersal, the other, to the south, has an uncertain origin. Placers are residual when left on the craton, transient when being eroded into the exit drainage, and terminal. Terminal placers, the final depositories of diamonds, have the greatest probability of being a mega-placer. There are four main groups of controls leading to the development of a mega-placer: the craton, the drainage, the nature of the environment at the terminus and the timing. Cratons, being buoyant, have a tendency to leak diamonds into surrounding basins; however, being incompressible they may have orogens converge onto them resulting in some lost sediment being returned as foreland basin fills. The craton size, its diamond-fertility and the retention of successive kimberlite intrusions that remain available to the final drainage, are significant to mega-placer development. Maximum potential recovery is achieved when the drainage delivering diamonds to the mega-placer is efficient, not preceded by older major drainages and focuses the supply to a limited area of the terminal placer. There should be sufficient energy in the terminal placer regime to ensure that sediment accompanying the diamonds is removed to areas away from the placer site. All conditions should be near contemporaneous and most were satisfied in the Orange-Vaal Rivers-Kaapvaal system and mega-placers were consequently generated.