Automotive emissions are severely regulated. Since 1980, a three-way catalyst (TWC) has been used to convert harmful emissions of hydrocarbons, carbon monoxide, and oxides of nitrogen into less harmful gases in order to meet these regulations. The efficiency of conversion of these gases is dependent on the mass ratio of air to fuel (A/F) in the mixture leaving the exhaust manifold and entering the catalyst, the velocity of the exhaust mass, and the temperature of the catalyst. The goal of this paper is to develop a dynamic, control-oriented model of a TWC. First, the importance of developing such a model will be explained. Then, a simplified dynamic catalyst model that can be determined on the basis of medium bandwidth A/F measurements and low bandwidth temperature and emission measurements will be introduced and validated.