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. For coatings the coating-substrate interface is equally important and other internal interfaces may also contribute to behavior. This article reviews the effect of interfaces on the hardness 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 is introduced which enables the effect of such interfaces to be more fully appreciated. The possibility of using graded compositions, such as ion implanted layers, to improve performance is also discussed in light of this model.