The principles by which acoustic and vibrational fields can be mimicked and cancelled by secondary sources are reviewed. Devices for exploiting this aspect of linear wave fields have been discussed and experimented with for some fifty years but it is only in the last decade that active noise control has emerged as a practical possibility. Known applications are reviewed, their performance is summarized and what constraints currently limit system performance are discussed. Developments are described that link the `anti-sound' problem with that of adaptive beam forming in antenna systems, and both deterministic and statistical criteria for optimal adaptation are discussed. These developments concern multidegree-of-freedom systems of wide bandwidth, and their application is strictly limited to linear fields. Some nonlinear fields may also be amenable to active control and the paper ends with some speculative discussion of the scope and significance of that area.