This annual Water-Data Report for Illinois contains current water year (Oct. 1, 2003, to Sept. 30, 2004) and historical data of discharge, stage, water quality and biology of streams; stage of lakes and reservoirs; levels and quality of ground water; and records of precipitation, air temperature, dew point, solar radiation, and wind speed. The current year's (2004) data provided in this report include (1) discharge for 177 surface-water gaging stations and for 10 crest-stage partial-record stations; (2) stage for 27 surface-water gaging stations; (3) stage for 8 reservoirs; (4) water-quality records for 7 surface-water stations; (5) sediment-discharge records for 16 surface-water stations; (6) water-level records for 14 ground-water wells; (7) precipitation records for 50 rain gages; (8) records of air temperature, dew point, solar radiation and wind speed for 1 meteorological station; and (9) biological records for 6 sample sites. Also included are miscellaneous data collected at various sites not in the systematic data-collection network. Data were collected and compiled as a part of the National Water Information System (NWIS) maintained by the U.S. Geological Survey in cooperation with Federal, State, and local agencies.