In air filtering applications, a filter's pressure drop at a defined collection efficiency constitutes an important parameter. This paper discusses the variables influencing the pressure drop in air filters featuring deep-pleated filter media. For cassette-type fine filters in accordance with EN 779 or for HEPA/ULPA filters in accordance with EN 1822, the most commonly used media are paper-like materials with a thickness of less than I mm, which offer a relatively high resistance to the air flowing through them. Manufacturers accordingly endeavour to accommodate a maximum of filter medium area in a small space. To enable the pressure drops customary in intake, exhaust and re-circulated air filtration to be assured, the filter medium is therefore arranged in narrow, deep pleats, Particularly when large quantities of air are being handled per filter element, it is advantageous to pleat the filter medium in depths of 150 mm to 280 mm. The conversion technique and the resultant pleat geometry exert a crucial influence on the pressure drop concerned.