We demonstrate the first successful side-by-side lateral wafer bonding of GaAs which is expected to increase the flexibility of the device fabrication process. First we give a brief review of compound semiconductor surface-to-surface direct wafer bonding, which has been remarkably successful in device applications owing to a new dimension in the fabrication process flexibility. Then results of side-by-side direct wafer bonding, which was achieved using a pair of cleaved facets taking advantage of their quasi-perfect flat surfaces, are presented. It is shown that submicron-order leveling can be easily achieved by face-down alignment of the two wafers. Lateral nn and pn junctions have been successfully formed using bulk substrates and even with epitaxial wafers. Current-voltage characteristics across the bonded interface show that no observable electrical barrier is formed by the bonding process. Although some oxides were found to exist at the bonded interface, TEM measurements revealed that atomic-order recrystallization is achieved in most of the interface after heat treatment at 700°C for 30 min.