The state of catalysts and filters plays a key role in automotive exhaust gas aftertreatment. The soot or ash loading of particulate filters, the oxygen loading degree of three-way catalysts, the amount of stored ammonia in SCR catalysts, or the NOx loading degree in NOx storage catalysts are important parameters. Today, they are determined indirectly and/or model-based, calibrated by gas sensors installed up- or downstream of the catalysts or by a differential pressure sensor.This contribution overviews a novel approach to determine directly the catalyst state by a microwave-based technique. For that purpose, the catalyst housing serves as a cavity resonator. As “sensing” element, one or two simple antennas are mounted in the catalyst canning. The electrical properties of the catalyst device, i.e., of the ceramic honeycomb incl. coating and storage material, can be measured. Such contactless-obtained information is very well correlated with the catalyst or filter state as is demonstrated for different exhaust gas aftertreatment systems.A comparison that calculates the relative sensitivity concludes this review.