The aim of this work is presentation of principles of work and properties of multimode interference (MMI) structures and their basic applications in optoelectronic circuits. We discuss the principles of simple, mirrored and multiple images formation paying attention to imaging quality, tolerance, losses, and power division in output signals. On the base of it, MMI applications in integrated optic systems - in splitters and couplers NxM technology, modulators and switches in Mach-Zehnder configuration and multiplexers are considered. The possibility of MMI production in gradient-index wavequides made by ion exchange in glass is also presented.