The delamination between metal and composite plies in FMLs is discussed for the two major crack opening modes and combinations thereof. The strain energy release rate is introduced as parameter to describe the static and fatigue behaviour, which is influenced by the interface geometry and fibre volume fraction of the composite plies. The relation between constant and variable amplitude loading is discussed. Furthermore it is explained how plasticity of the metal layers affects the quasi-static delamination fracture toughness. Some observations concerning delamination buckling and residual stress state are discussed.