The previous chapter covered factors affecting strength, in terms of the stresses at which damage and failure occur in composites. In many situations, however, it is the energy that is absorbed within the material while fracture takes place that is of prime importance. A tough material is one for which large amounts of energy are required to cause fracture. Some loading configurations, such as a component being struck by a projectile, provide only a finite amount of energy that could cause failure. In fact, there are many situations in which toughness, rather than strength, is the key property determining whether the material is suitable. In this chapter, a brief outline is given of the basics of fracture mechanics, with particular reference to the energetics of interfacial damage. This is followed by an appraisal of the sources of energy absorption in composites. Finally, progressive crack growth in composites is examined under conditions for which fast fracture is not energetically favoured (sub-critical crack growth).