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More than 30 years ago, the Metropolitan Water Reclamation District of Greater Chicago (District) determined that the dissolved oxygen (DO) standards established for deep-draft waterways in the Chicago metropolitan area could not be continuously met by implementing advanced wastewater treatment and through the capture and treatment of combined sewer overflows (CSOs). In order to increase the DO concentration in these urban deep-draft waterways, the District designed, constructed, and began operating seven supplemental instream and sidestream aeration stations during the late 1970s and early 1990s, respectively.From 1994 through 1996, a field monitoring program was initiated in order to determine the effectiveness of the supplemental aeration stations on DO in the deep-draft waterways in the Chicago metropolitan area. Weekly DO surveys were conducted at 75 stations. The DO was measured by manually collecting water samples. In the laboratory, the water samples were titrated using the Winkler Method. The monitoring data showed that even with supplemental aeration, DO values at selected stations were still below water quality standards.Shortly following the DO stream surveys, the District developed a new monitoring approach for assessing DO impairment in urban waterways. The methodology was the use of real-time, continuous water quality monitors (monitors). Beginning in August of 1998, DO was measured hourly at 20 stations on the deep-draft waterways using remote in-situ monitors. Fifteen additional stations were later added to the waterway DO monitoring network. Continuous DO monitoring will continue through 2003.The primary objective of the continuous DO monitoring program is to identify DO impairment in Chicago area waterways. The real-time continuous DO data will also be used for calibrating and verifying a non-steady state water quality model and for future TMDL and UAA studies.The paper will discuss the continuous DO monitoring approach used in urban deep-draft waterways in Chicago. Included in the paper are (1) the criteria used for selecting monitoring stations; (2) the features of the continuous monitors; (3) a description of the housing enclosures used to protect the monitors in the field; (4) the field and laboratory DO monitoring programs; (5) the quality control program and the data management system; and (6) examples of the continuous DO monitoring data showing the causes and sources of DO impairment in Chicago urban waterways.