The most significant property improvement in fibre reinforced composites is that of fracture toughness. Toughness is quantified in terms of the energy absorbed per unit crack extension and thus any process which absorbs energy at the crack tip can give rise to an increase in toughness. In metallic matrices, plastic deformation requires considerable energy and so metals are intrinsically tough. In fibre reinforced materials with both brittle fibres and brittle matrices, toughness is derived from two sources. Firstly, if the crack can be made to run up and down every fibre in its path the there will be a large amount of new surface created for a very small increase in crack area perpendicular to the maximum principal stress - INTERFACIAL ENERGY - and in order to get the fibres to break they have to be loaded to their fracture strength and this often rquires additional local elastic work, and secondly If the fibres do not break and therefore bridge the gap then work must be done to pull the fibres out of the matrix - FIBRE PULLOUT. Using simple geometric models we can estimate the contribution of each of these processes to the overall toughness of the composite.