Honeycomb substrates, often simply called “honeycombs”, are the basic geometric form of modern fixed-bed catalysts. They usually consist of extruded ceramic (e.g. cordierite) or corrugated metal foil. The honeycomb structure offers an optimum balance between a very large active surface area and minimal exhaust back pressure. The exhaust gas flows through the fine channels, where the pollutants come into contact with the coated channel walls. The cell density (stated in cpsi, cells per square inch) is selected according to the dust load and the required cleaning efficiency. In dusty exhaust gases, coarser honeycombs are used to prevent clogging, while in clean gas streams very fine honeycombs ensure maximum efficiency. Honeycomb substrates are mechanically stable and can withstand high thermal loads, which makes them the standard in industrial exhaust gas cleaning.