The report addresses the quality of the waters of the State of Illinois for the period of 1984 and 1985. Water quality indices and percent violations of State water quality standards were computed for ambient stream data by river basin. Intensive survey results are given for selected basins and were used to assess stream use attainment. Existing and potential biotic resource characterizations of Illinois streams are given. Data also summarized included organics in water and fish flesh, bottom sediment, and facility-related surveys. Ambient and volunteer lake and Lake Michigan water quality data were analyzed, and intensive lake/watershed surveys (including Section 314 Clean Lakes Program diagnostic/feasibility studies) were summarized.