In studies of environmental quality, there are several different groups of bacteria frequently used as fecal indicators. The most widely applied approach is to use the group represented by the family Enterobacteriaceae (coliforms and fecal coliforms). Fecal indicators other than Escherichia coli, include members of the genus Enterococcus (formally fecal streptococci) and the anaerobic bacteria Clostridium perfringens, Bifidobacteria, and some members of the genus Bacteroides. Due to their widespread use as indictors of the quality of water and other environments, this chapter covers the phylogeny, taxonomy, and physiology of the key fecal indicator bacteria. E. coli and K. pneumoniae are specifically classified as fecal coliforms. The major set of phenotypic characteristics defining the Enterobacteriaceae family include the following: small gram-negative rods, facultative but preferring aerobic metabolism, oxidase negative, catalase positive, can reduce nitrates to nitrites, do not require Na+ for growth, can ferment D-glucose, and contain enterobacterial common antigens. Originally, classification of coliform bacteria was based on fermentation of sucrose and dulcitol, production of indole and acetylmethylcarbinol, and gelatin liquefaction. Functionally, enterococci are divided into five groups based on their ability to produce acid from mannitol and sorbose, and hydrolyze arginine. Bacteroides spp. are present in much greater numbers than E. coli, and can account for up to 30% of the total fecal isolates, with the most common species being B. vulgatus, B. distasonis, and B. thetaiotaomicron.