Wetlands are complex ecosystems that support wildlife and provide many important ecosystem services. This article provides information on the state of Illinois wetlands through the use of data collected by the Critical Trends Assessment Program (CTAP). CTAP is an ongoing statewide survey of Illinois' natural resources that has been collecting data in Illinois wetlands since 1997. Topics that are covered in this article include: Descriptions of Illinois wetlands and their history, trends in wetland plant and arthropod communities, invasive species in Illinois wetlands, encroachment of woody plants into herbaceous wetland communities, and climate change. Recommendations for the conservation of wetlands in Illinois are also provided.