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Active control can reduce sound and vibration by destructive interference between the original, primary, field and the field generated by controllable secondary sources. It is most effective at low frequencies, for which the wavelength of the disturbance is comparable with the dimensions of the region being controlled. In this review paper the physical limitations of performance are first explored for vibration on a plate and sound in an enclosure, and the nature of the plant response is discussed in these two cases. ; Adaptive feedforward control algorithms have been successfully used in active control, and these are discussed in terms of an equivalent feedback control system. Finally, the feedback control of sound and vibration is discussed, from the point of view of Internal Model Control, emphasising the importance of plant delay in determining optimal performance, and the trade-off between disturbance rejection and robust stability.