The properties and performance of most bulk materials and coatings are controlled by interfaces. For bulk materials the surface is the most important interface, though grain and phase boundaries may also play a role in dictating performance. However, when developing coating systems there is the possibility of using many interfaces to control behaviour. As well as the surface, the coating/substrate interface will be important and in multilayer coating designs the number and properties of the individual layer interfaces will also play a role. It has often been observed that multilayer coatings offer enhanced hardness and fracture resistance compared to comparable single layer coatings though the reasons for these improvements are not always well understood. This paper will review the effect of interfaces on the mechanical properties and tribological performance of bulk materials, single and multilayer coatings highlighting the importance of engineering interfaces with high strength if coatings with optimum properties are to be achieved. A simple model will be introduced which enables the effect of such interfaces to be more fully appreciated. The possibility of using graded compositions to improve performance will be discussed in light of this model.