This paper outlines a series of tests on vibrations in knee-type horizontal milling machines, especially in the knee system. The amplitude of the vibrations was determined by means of a capacitive pickup. Systematic investigations on the importance of clamping and unclamping the various elements are made. The influence of damping in the case of resonance is pointed out. An explanation of the vibration accompanying entrance and exit of the milling cutter is indicated. A theory for the presence of low natural frequencies in the knee system is developed. Up- and down-milling are compared.