There are numerous potential indicators of distribution system contamination, some of which may be useful for detecting unexpected contamination of the distribution system. Microbial indicators include total coliforms, fecal coliforms, E. coli, enterococci, fecal streptococci, somatic coliphage, f-specific coliphage and heterotrophic bacteria. Various chemical (e.g, sterols, turbidity, AOC) and physical indicators (e.g, water loss, pressure drops) may also have value in detecting distribution system contamination. The purpose of this paper is to examine the available information on these indicators. The strengths and weaknesses of each indicator considered will be identified in terms of its utility as an indicator of: distribution system integrity, including issues such as degradation of the infrastructure, cross-connections, and excess biofilm growth; fecal contamination, including a range of issues with the potential to allow fecal contamination to enter the distribution system; toxic or carcinogenic chemicals, examining indicators of issues such as corrosion and leaching; and, waterborne disease outbreaks and endemic disease, examining a range of issues that could be detected as a result of waterborne disease. Includes abstract only.