This article presents the results of a study designed to evaluate the use of pathogen surrogates as a tool in treatment optimization and to predict pathogen occurrence. A comprehensive matrix was developed to relate water quality and process variables for 24 potable water sources and 12 possible surrogates to the target pathogens of Giardia, Cryptosporidium, and enteric viruses. No "ideal" surrogate for predicting occurrence of protozoan pathogens or treatment performance was identified because of analytical constraints and low pathogen occurrence. However, it was concluded that aerobic spores and particle counts could be used as surrogate measures to evaluate or improve treatment plant performance. Utilities should select not one but a number of surrogates to assess raw water quality and treatment plant performance. Includes 28 references, tables, figures.