N,N-Diethyl-m-toluamide's (DEET) commercial use as an insect repellent and other reported uses are reviewed. Evidence that DEET is reaching the environment mainly from consumer use of DEET-containing insect repellent includes studies reporting higher concentrations of DEET in surface water and wastewater samples during the summer months, the presence of DEET in on-site septic tank effluent at concentrations similar to that reported in wastewater treatment plant (WWTP) influent, and changes in WWTP effluent concentrations before and after the introduction of a DEET replacement in Germany. Its detected concentrations in influent and effluent of WWTP and surface water worldwide are reviewed and correlations between DEET usage and wastewater effluent concentrations are analyzed. The removability during wastewater treatment is also evaluated. A correlation between commercial DEET use in a metropolitan area and concentrations in WWTP effluents was assessed, and 2 different models were used to predict DEET concentrations in rivers and streams throughout the United States. Ecological toxicity data are reviewed for acute studies and for chronic values that are available for Daphnia magna and algae. The ecological risk of DEET usage is evaluated by examining the relationship of the expected dose/response to observed concentrations.