Lake Okeechobee, Florida, is managed for multiple uses, including drinking water supply, recreation, wildlife habitat, and flood control. Because much of the drainage basin is in agricultural production, maintenance of water quality conditions is a major management consideration for the lake. An evaluation of water quality data collected over a 20-year period indicated that Lake Okeechobee and its major inflows generally comply with applicable numerical criteria, and that parameters that exceeded criteria at inflows have not impaired die designated uses of the lake. Of 59,781 water quality measurements, 6,409 (10.7 percent) exceeded the applicable criteria. For most monitored parameters, values that exceeded criteria generally represented less than 25 percent of die measurements at specific locations. Exceptions included dissolved oxygen (DO) andiron. Low DO values (>5.0 mg·L−1) represented 9 to 100 percent of measurements at inflow stations, although fewer than one percent of the observations at any in-lake station were below die criterion. Iron values were commonly higher than die applicable criteria at many inflow and in-lake stations. However, most of die high iron values were greater than the secondary drinking water standard, and very few were greater than the criterion for protection of aquatic biota. Trace metals (including mercury) and organic contaminants were rarely greater than die criteria in the lake and its inflows, in spite of die predominance of agriculture in die basin. Pesticide application methods, degradation processes and dilution appear to be responsible for the generally low concentrations of pesticides and herbicides in Lake Okeechobee and its inflows. Many of the criteria that were exceeded are drinking water standards that do not relate to protection of aquatic biota. Because of water treatment processes, they do not appear to have affected die suitability of the lake as a potable water supply. Additional monitoring of biological conditions, in conjunction with continued monitoring of selected water quality parameters, is recommended to document whether die lake and its inflows meet their designated uses.