The fiber-matrix interface is believed to play a central role in the load-supporting and crack-growth resisting capabilities of fiber-reinforced high temperature composites. In metal matrix composites, this interface can be very complex: new phases may be present due to reactions between the matrix and reinforcement, compositional gradients due to interdiffusion occur, and the outer fiber surface is usually coated to avoid handling damage, improve fiber wetability, and/or control matrix reactivity. Attempts at measurements of fiber and interface strength from studies of actual composite samples are thwarted in part by the difficulty of determining the local distribution of stress and strain in these highly inhomogeneous materials. Furthermore, the inability to determine the point, during loading, where failure occurs in the interfacial zone also complicates the problem in these opaque materials. It is not surprising, therefore, that the fundamental relationships between the structure and properties of interfaces, on the one hand, and the load-supporting/crack-resisting properties of bulk metal matrix composities, on the other, have not been determined to date.