Turbidity is an optical measure of the concentration of suspended particles in water. Drinking water producers make extensive use of continuous turbidity measurements to control water treatment. The turbidity of treated water is the primary proxy for exposure to fecal pathogens in the multicenter time-series study "Turbidity and Gastroenteritis." The utility of raw water turbidity is also being tested in this study. This document reviews the theoretical foundations of this measurement, its practical methods, and its standards, and describes the criteria and quality indicators for the measurements. It then focuses on data from continuous measurements and their correction, and outlines specifications for the quality of measurements intended for epidemiological use. (R.A.)