The interest in high-rate anaerobic (pre-)treatment of sewage using UASB reactors is steadily growing since its introduction in the mid-1980s. Today there are hundreds of full-scale plants in operation in various parts of the tropical world, notably in Latin America and India. The main advantage of UASB technology is the very low or even zero energy demand, leading to an up to tenfold drop in operational costs compared to activated sludge. This paper presents a literature review focussing on current design criteria and post-treatment options, alongside discussing the centralized and decentralized approach. The current limitations and constraints regarding temperature, nutrients, pathogen removal, odour nuisance, operational constrictions and methane emissions are also presented and discussed. Further, recent challenges in relation to energy recovery from biogas, sludge and scum are discussed, alongside with advances related to recovery of dissolved methane and sludge management. Finally, the paper provides some outlooks for upcoming developments.