This document provides users with information that assists them in specifying, installing, calibrating, maintaining and monitoring the subsequent performance of on-line residual chlorine analyzers in wastewater treatment plants. An on-line residual chlorine analyzer must have, among other characteristics: capability for continuous operation; a sampling system that assures a representative sample with the proper contact time; range of from 0-1 to 0-10 mg/L of chlorine residual depending upon the application; accuracy and precision within 3 percent and 1 percent of range, respectively; response time less than three minutes; and on-site readout and output capability for transmission to a controller and/or recorder. Calibrations should be based on amperometric back titration and should be performed at frequencies that are based on documentation obtained during startup. Methods for developing this information and other procedures for measurement assurance are discussed. Recommended maintenance guidelines are given.