The successful utilization of single-bed three way catalysts is dependent upon high initial catalyst activity and maintenance of this activity after exposure to a wide range of exhaust composition and temperature. A review of performance results on three-way catalysts after aging under a variety of engine dynamometer and thermal cycles indicates high temperature aging is particularly detrimental to CO conversion while HC conversion is most sensitive to chemical poisoning. Results on individual Platinum Group Metal Catalysts show Platinum has good chemical poison resistance, but is sensitive to thermal deactivation. Palladium shows good thermal stability, but exhibits more deactivation by chemical poisoning. Rhodium is significantly deactivated by lean high temperature aging. These results have been used to compare and contrast the deactivation of bimetallic and trimetallic catalyst formulations. A new generation of catalysts has been developed utilizing the above information. These catalysts exhibit substantially improved performance and thermal stability.