A frequent problem in the manufacturing industry today is the vibrations or chatter induced in machine tools during machining. Chatter in machine tools affect the result of machining, particularly the surface finish. Furthermore, tool life is correlated with the degree of vibration and acoustic noise introduced. The material deformation process during turning induces a broadband excitation of the machine tool and chatter or tool vibration is usually related to a low-order bending mode of, for example, the tool holder shank in external turning, the boring bar in internal turning. Frequently, the tool holder responses during continuous unstable machining have non-linear properties and exhibit advanced stochastic behaviour. Generally, the tooling structure - the interface between the cutting tool or insert and the machine tool - is the weakest link in a machining system. Active control of tool vibration, however, enables a flexible solution to selectively increase the dynamic stiffness of the tooling structure at the actual frequency of the dominating bending modes. Based on adequate controllers, tool holder shanks and boring bars with embedded actuators and vibration sensors tool vibration in turning may be reduced significantly.