A catalog of water-related data collected in Illinois was developed as part of a monitoring strategy review to serve as a reference for data availability and data collection efforts in the State, and therefore to facilitate monitoring decisions and evaluations. A questionnaire was sent out to various organizations to gather complete and up-to-date information directly. Information requested included: number of stations, parameters and collection frequency for several data categories, divided into stream, lake and groundwater sections; general program information; and computer, reporting and cooperative. The Illinois Water Data Catalog summarizes the returns by organization in table format.