This paper presents an innovative product for emission control: the Modular Block Metallic Substrate.Pre-coated metallic foils are welded to a metallic frame and form a substrate block (typically 100 cm3). This allows a standardized manufacturing process, which is independent of the block cell density.The blocks are welded together to form the converter, the number of blocks assuring the required substrate volume. This modular approach results in high design flexibility allowing the use of high cell density substrates (up to 1250 Cpsi) without any backpressure increase. It also allows dealing successfully with severe space constraints.The paper describes the principles of design and manufacturing of the Modular Block Metallic Substrate and demonstrates the durability performances of the blocks at high temperatures and vibration levels. Examples of applications are presented to show the advantages of the concept when high design flexibility is required. Finally flow calculations and emission performances are compared to more conventional designs.