The reduction of emission levels for low (LEV) and ultra low (ULEV) emissions vehicles requires catalyst with rapid thermal response for quick light-off while providing stable high temperature performance under highway driving conditions. The design of substrates which carry the three-way catalyst formulations are key to meeting the complex dual performance requirements. Enhanced mass transfer and reduced thermal mass can be accomplished while maintaining acceptable back pressure. The design principles and selection of substrate properties are described as is their effect on the emissions from the Federal Test Procedure. Transient driving performance simulations allow for selection of optimum advanced substrate designs.