The measure of a successful air filter seal is its ability to seal at installation, and reseal after routine inspection, without exerting excessive force on the air cleaner housing. Filter seal evaluation requires selection of a test sample to measure, a property to measure, and a measuring method. Test samples include complete filter seals, portions of a filter seal, slab samples of various shapes, and free-rise cup samples. Properties include compression set and resistance to compression. Measuring methods include clamping samples between plates, durometer readings, and in-housing compression tests. The merits of each test sample, property, and measuring method will be discussed with regard to microcellular polyurethane foam seals.