The application of modern engineering materials often requires that two different materials be bonded. The resultant interfaces must typically sustain mechanical and/or electrical forces without failure. Consequently, interfaces exert an important and sometimes controlling, influence on performance in such applications as composites, electronic packaging systems, thin film technology and joining.During joining by a diffusion-bonding process, surfaces of the two materials are brought into contact by applying a small load and temperature. During the bonding and the subsequent cooling cycle chemical reactions may occur leading to chemical gradients or chemical reactions. The actual reaction and diffusion paths depend on the activities of the diff erent elements present near the interface.Bonds of bulk Nb/Al2O3 and Ni/Al2O3 were formed (for details of bonding conditions, see Elssner et al. ) and structure of the interface as well as chemical gradients near the interfaces were studied by TEM, HREM and SEM.