The mechanisms by which delamination contributes to the failure of fibre-reinforced composites are reviewed. Through-thickness failure owing to interlaminar stresses is considered first, and the effect of delamination in impact and compression after impact. The way in which in-plane failure can occur by delamination and matrix cracks joining up to produce a fracture surface without the need to break fibres is considered next. Examples of quasi-isotropic laminates loaded at different off-axis angles, and with different numbers and thicknesses of ply blocks show large differences in unnotched tensile strength controlled by delamination from the free edge. Similar mechanisms determine the strength of notched specimens and give rise to the hole size effect, whereby tensile strength increases with decreasing hole diameter owing to increased delamination and splitting. Open hole tension and over-height compact tension tests with constant in-plane dimensions show a transition in failure mode with increasing ply block thickness from fibre-dominated fracture to complete delamination. In all these cases, the critical factor controlling strength is the relative propensity to delaminate.