This chapter describes procedures for microbiological examination of environmental samples to determine sanitary quality. The methods are selected according to the best techniques currently available; however, their limitations must be understood thoroughly. Examination of routine microbiological samples cannot be regarded as providing complete information concerning water quality. The test provides an approximate enumeration of total numbers of viable bacteria that may yield important information about microbiological quality of the sample and may provide supporting data for the coliform test results. Indicator bacteria, such as coliform and fecal streptococci, may become stressed or injured by other disruption. The stress may be caused by temperature changes or chemical treatment, such as chorine or toxic wastes, especially toxic metals and phenols. Water samples from natural waters at high temperatures may include large number of noncoliform organisms which interfere with sheen-production on membrane filter (MF) and with positive gas production in most probable number (MPN) analysis.