The integration of materials by wafer bonding offers novel device fabrication for applications in micromechanics, microelectronics, and optoelectronics. Two mirror-polished surfaces are brought into intimate contact by adhesive forces regardless of their crystallography, crystalline orientation and lattice mismatch. Followed by a thermal treatment at several hundred degrees centigrade, the interface energy of the material combination is increased to energies of covalent interatomic bonds. Attempts to break the bond lead to fracturing of the materials. In particular, thermomechanic stress in dissimilar material combinations may result in bending, gliding and cracking of the bonded wafers during annealing. The bonding interface of various hybrid semiconductor materials was studied by transmission electron microscopy. Occasionally, microscopic imperfections at the bonding interface were found in Si/Si, Si/GaAs, GaAs/GaAs, GaAs/Al 2O 3, GaAs/InP and moreover Al 2O 3/Al 2O 3 bonded wafer pairs. The imperfections were identified as voids, negative crystals, and oxide-containing precipitates ranging from 5 to 20 nm in diameter. Microscopic defects at the bonding interface in integrated bulk materials do not affect the mechanical and electrical properties of the device very much. However, in bonding of thin films the defects or precipitates may thread through the thin film, if the diameter of the precipitate surpasses the thickness of the film. These pinholes-containing thin films have a high leakage current, low electrical breakthrough and crystallographic disorder. Epitaxy of material on a pinholes containing, disordered surface results on deposition of bicystalline grains. In between the grains tilt grain boundaries were observed raising from the bonding interface. Bonding related defects at the interface can be avoided by alternative bonding techniques like UHV wafer bonding and low temperature wafer bonding.