IDEM is the primary state agency with Clean Water Act (CWA) authority to implement programs to protect and restore water quality. IDEM employs three different monitoring approaches including a fixed station network where sites do not change from year to year; a probabilistic approach, which is a stratified random method to site selection; and a targeted approach where the selection of sampling sites is based on specific monitoring objectives or decisions to be made. Since the focus of this paper is to identify long-term water quality networks, the fixed station network (FSN) is highlighted. IDEM’s FSN, which began in 1957 with 49 sites located primarily at drinking water intakes and wastewater treatment plant outfalls, has expanded to 165 sites that are sampled monthly for nutrients, major ions, metals and other field parameters (Appendices 1 and X). For this paper, only 156 sites are included as the other nine sites are on canals or lakes. The original purpose of the FSN was to determine point source compliance and mixing zone effects. For the last decade or so, the data are primarily used to calibrate and verify waste load allocation models for National Pollutant Discharge Elimination System (NPDES) permits and, more recently, to determine water quality trends.