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Ultrasonic mechanical vibrations are usually introduced perpendicular to the welded surfaces, which coincides with the direction of the clamping force. The characteristics of the physical processes in ultrasonic welding (USW) of polymers cause heating of the parts to occur primarily at the site of their contact. For this reason, it is possible to form a high-quality weld at a lower temperature than with other kinds of welding. In some cases, USW of polymer parts is even possible at a temperature below their pour point (melting point), which allows obtaining welded articles made from different polymers and welding polymers whose degradation temperature is comparable to or lower than the melting point [fluoroplastic, poly(ethylene terephthalate)]. One of the most important characteristics of USW is the possibility of forming a high-quality weld at a relatively large distance from the site of introduction of mechanical energy, which allows obtaining sufficiently strong welded joint in articles of very complicated design. The possibility of manufacturing welded joints of polymers with contaminated surfaces significantly expands the range of application of USW, as it can be used for hermetic sealing of polymer containers filled with finished products. USW allows industrial production of nonwoven cloth both from purely thermoplastic polymer fibers and from blends with natural or other chemical fibers.