The impact of biofilms present in water distribution systems on the microbial quality of potable water is reported in this review. The issues covered include the composition of biofilms, factors governing their formation and the effect and significance of biofilms on the microbial quality of drinking water. The review addresses the main factors governing the formation of biofilms such as the types of disinfectants and residual concentrations, resistance of bacteria to disinfectants, the influence of piping material and the effect of temperature. Methods for the enumeration of bacteria in biofilms as well as emerging technologies for in situ monitoring of biofilms are discussed. Suggested control measures for managing and controlling the problem of biofilm formation in potable water distribution systems to ensure potable water of an acceptable microbiological quality are dealt with.