The establishment of microbiological contaminant limits for water and wastewater and the implementation of enforcement programs which require monitoring of these limits have emphasized the need to develop and practice quality assurance (QA) programs. A broadscope QA program is essential for microbiological determinations. The basic elements of an intralaboratory quality control (QC) program are described, which include controls over personnel, facilities, sampling techniques, laboratory operations, analytical procedures, and data reporting. Formal, written analytical procedures, a QA protocol, and complete QC records are indispensable to a QA program. The laboratory should participate in an interlaboratory QC program established to maintain minimal analytical and operational standards. The U.S. Environmental Protection Agency (EPA) certification program for analyzing public drinking water supplies is described as an example of a national interlaboratory program. Guidelines provided by the EPA for laboratory QA programs are outlined.