Biofilms in drinking water pipe networks can be responsible for a wide range of water quality and operational problems. However, one of the main reasons that many utilities become concerned with biofilms is due to the growth of coliform bacteria within the pipe network. This paper summarizes the results of recent studies that have examined data from over 90 water systems to determine the factors that contribute to the occurrence of coliform bacteria in drinking water. The studies have shown that the occurrence of coliforms can be related to the following factors: filtration; temperature; disinfectant type and residual; assimilable organic carbon (AOC) level; corrosion control; and, pipe material selection.