Sludge flotation is a commonly reported and long-standing issue hindering not only the widespread implementation of upflow anaerobic sludge bed (UASB)-type bioreactors in wastewater treatment but also the development of novel anaerobic/anoxic treatment processes such as anammox, partial denitrification, and biological sulfate reduction. This review attempts to address the instability of UASB-type bioreactors due to sludge flotation. Possible causes of sludge flotation are classified into intrinsic and extrinsic ones. Extrinsic causes include substrate overloading, inappropriate carbon source, overloading of proteins or oils, insufficient reactor mixing, a low temperature, and a low pH. These unfavorable extrinsic conditions can lead to unexpected intrinsic changes in sludge granules, including high gas production, formation of hollow space inside the granules, filamentous bacterial overgrowth, inappropriate production of extracellular polymeric substances, and development of an adhesive granule surface. These intrinsic changes can increase the flotation potential of sludge through reducing the granule density and promoting gas entrapment. To control the sludge flotation problem, both preventive and corrective strategies are summarized. Preventive strategies include maintaining a temperature of 30-35 A degrees C and a pH of 7-9, preventing substrate overloading, providing sufficient nutrients and multiple carbon sources in the influent, applying pre-acidification, and enhancing reactor mixing. If the causes of a sludge flotation incident cannot be identified quickly, corrective strategies including breaking up floating granules and dosing with chemicals such as Fe2+ and surfactants can be applied to suppress the flotation problem.