AbstractA review of the toughness properties of metal matrix composites (MMCs) is presented. It is shown that the toughness of MMCs is controlled by a complex interaction between the properties of the constituent phases and the processing route and that no single parameter is capable of describing all the features of their toughness response. It is proposed that the toughness of MMCs is best characterised by a combination of the fracture toughness and the energy values associated with the initiation and propagation of fracture. This approach is used to illustrate methods of optimising toughness in MMCs.MST/828