This paper provides an overview of the dual EGO sensor method for OBD-II catalyst efficiency monitoring. The processes governing the relationship between catalyst oxygen storage, HC conversion efficiency, and rear EGO sensor response are reviewed in detail. A simple physical model relating catalyst oxygen storage capacity and rear EGO sensor response is constructed and used in conjunction with experimental data to provide additional insight into the operation of the catalyst monitor. The effect that the catalyst washcoat formulation has in determining the relationship between catalyst oxygen storage capacity and HC conversion efficiency and its impact on the catalyst monitor is also investigated. Lastly, the effects of catalyst failure mode, fuel sulfur, and the fuel additive MMT on the catalyst monitor's ability to properly diagnose catalyst function are discussed.