This report presents a summary of air quality data collected throughout the State of Illinois during the calendar year - 2004. Data is presented for the six criteria pollutants (those for which air quality standards have been developed - particulate matter (PM10 and PM2.5), ozone, sulfur dioxide, nitrogen dioxide, carbon monoxide, and lead) along with some heavy metals, nitrates, sulfates, volatile organic and toxic compounds. Monitoring was conducted at over 80 different site locations collecting data from more than 200 instruments. In terms of the Air Quality Index (AQI) air quality during 2004 was either good or moderate more than 98 percent of the time throughout Illinois. There were no days when air quality in some part of Illinois was considered Unhealthy (category Red). There were 7 days (all for PM2.5 ) when air quality in some part of Illinois was considered Unhealthy for Sensitive Groups (category Orange). This compares with 19 Unhealthy for Sensitive Groups days in 2003. In fact 2004 marks the first year since monitoring has begun that there were no 8-hour ozone days recorded above the standard. Air quality trends for the criteria pollutants are continuing to show downward trends or stable trends well below the level of the standards. Percentage changes over the ten year period 1995 2004 are as follows: Particulate Matter (PM10) 10 percent decrease, Sulfur Dioxide 31 percent decrease, Nitrogen Dioxide 15 percent decrease, Carbon Monoxide 45 percent decrease, Lead 31 percent decrease, and Ozone 9 percent decrease. Stationary point source emission data has again been included. The data in the report reflects information contained in the Emission Inventory System (EIS) as of December 31, 2004. Emission estimates are for the calendar year 2004 and are for the pollutants: particulate matter, volatile organic material, sulfur dioxide, nitrogen oxides and carbon monoxide. Emission trends of these pollutants has been given for the years 1995 to the present. Emissions reported with the Annual Emissions Report have been provided starting with 1995 and are currently available through 2003. In general there has been a trend toward decreasing emissions over this time period.