Coliform occurrences in distribution systems have created a great concern for both utilities and water authorities because of the implied public health implications and failure to meet Federal regulations. Many of the known cases involve systems in the east and midwest. The common denominator being systems that have significant amounts of pipe networks over 75 years old and all are treating surface waters. Origins for these contamination events can be found in source water fluctuations, failures in treatment barrier protection or loss of pipe network integrity. Once passage into the distribution network has been achieved, some of the coliforms (Klebsiella, Enterobacter, Citrobacter) and other heterotrophic bacteria adapt to the pipe environment, finding protection and nutrient support in pipe sediments. Under conditions of seasonal warm waters (10 degC) and availability of assimilable organics in the pipe sediments and tubercles, colonization grows into biofilms that may slough-off into the water supply, creating a coliform non-compliance problem. Significance of these occurrences and control measures are part of a realistic action plan presented for guidance.