The Interim Enhanced Surface Water Treatment Rule willtake effect on December 17, 2001. Water systems treating surface water or groundwater under the influence of surface water and serving 10,000 or more persons will have to comply with the Rule. The principal aspects of this rule that will affect filter operation and maintenance are the tightening of the filtered water turbidity performance standard and the requirement for monitoring of turbidity of individual filters. More and more reliance is being placed on instrumentation and electronics in the monitoring and operation of water filtration plants. Decisions on water treatment are being made on the basis ofinstrument read-outs or electrical signals transmitted from a remote sensor to a computer screen or data panel in the control room or a similar place in the treatment plant. Carrying out an effective quality control program is the key to being able to trust the instruments and the information they are providing. This paper discusses monitoring with on-line tubidimeters; their installation, inspection, maintenance, and calibration. Filter ripening and techniques to control the initial turbidity spike of filtered water are also addressed. Includes 22 references, tables.