The U.S. Environmental Protection Agency's (EPA) Office of Toxic Substances evaluates existing chemicals under Section 4 of the Toxic Substances Control Act (TSCA) and Premanufacture Notification (PMN) submissions under Section 5 of TSCA. Azo dyes constitute a significant portion of these PMN submissions and specific azo dyes have recently been added to the priority list for considerations in the development of test rules under Section 4. Azo dyes are of concern because some of the dyes, dye precurors, and/or their degradation products such as aromatic amines (which are also dye precurors) have been shown to be, or are suspected to be, carcinogenic. The fate of azo dyes in biological wastewater treatment systems was studied to aid in the review of PMN submissions and to assist in the possible development of test rules. Results from extensive pilot-scale activated sludge process testing for 18 azo dyes will be presented. Results from fate studies of C.I. Disperse Blue 79 in aerobic and anaerobic wastewater treatment will also be presented.