The jar test has been and is an important drinking water treatment plantdesign, process control, and research tool. In the drinking water field,the jar test is described as a bench-scale simulation of full-scale coagulation/flocculation/ sedimentation water treatment processes. The jar test procedure is site specific and no standard jar test protocol exists. Results from a jar test are particularly valuable to the plant operator in making adjustments to treatment process(es) in reaction to water quality changes. This paper describes features of the jar test apparatus and presents a general outline of a jar test procedure. Factors that are critical to producing consistent and representative results are also discussed.