Compression seals are essential functional elements of modern windows and exterior doors. They are inserted into grooves on the fixed or sash frame and are usually made of EPDM, TPE or silicone. When the sash closes, the seal deforms elastically and hermetically closes the gaps between the movable and fixed frame profiles. Multiple sealing planes improve classifications according to EN 12207 (air permeability) and EN 12208 (watertightness). Typical profiles are co-extrusions with a hard carrier and a soft sealing lip. Longevity depends on UV resistance, temperature cycling resistance and compression set. Age-related hardening leads to leakage and can be delayed by regular maintenance with silicone grease.