Water storage tanks and reservoirs are a critical component of distribution systems, yet they can pose a significant challenge for water utilities as they often have a negative impact on water quality. This paper discusses the science of mixing applicable to all styles of storage tanks including circular and rectangular reservoirs, standpipes, and elevated tanks. Mixing characteristics of storage tanks are quite complex due to the closed geometry of the tank, the effect of inlet flow momentum, and the effect of buoyancy differences caused by temperature differentials. Computational Fluid Dynamics (CFD) and Physical Scale Models are utilized to illustrate the effect of various inlet configurations, inlet momentum, and temperature differentials on the mixing processes within storage tanks. The modeling results demonstrate how water quality problems develop, and how they can be corrected with a properly designed piping/mixing system.