Mathematical modeling has been an important tool in evaluating CSO abatement alternatives for Onondaga County, NY. The EPA Stormwater Management Model (SWMM) has been used to simulate the hydrologic and hydraulic elements of the combined sewer system. The modeling applications began with CSO facility planning in the late 1980’s and have evolved into more detailed applications for the ongoing planning and design phases of CSO projects. In addition, the models were instrumental in obtaining regulatory acceptance of the CSO Program. The models were calibrated and used to project the effects of a range of abatement alternatives. In addition, the models have been used to project specific design storm conditions (e.g., one-year and two-year design storms) as well as annual discharge volumes using continuous (long-term) simulation. The models were successful in identifying the maximum conveyance capacity of the collection system. This as an important element used in determining the basis of design for the entire CSO abatement plan which was the one-year storm. Long-term simulation was also used to determine the annual discharge volumes before and following the build-out phases of the program. The results of the modeling showed that following abatement, 85% of the combined sewage would be captured for primary treatment at the Metropolitan STP. Although the CSO program was primarily developed to meet water quality requirements, the 85% capture was an important element in gaining regulatory approval for the CSO program. As a result, the program will meet both the “demonstrative” and “presumptive” clauses of the Federal CSO policy.