This paper presents an overview of integrated optics outline of the modal propagation analysis (MPA), which will be used later to describe image formation by general and restricted multimode interference (Sections IV and V, MMI devices are discussed in Section V I . Performances and compatibility with other components are presented through examDles of fabricated MMI CouDlers and their aDDlications routing and coupling devices based on multimode interference. The underlying self-imaging principle in multimode waveguides is described using a guided mode propagation analysis. Special ference devices are discussed, followed by a survey of reported applications. It is shown that multimode interference couplers Offer Superior performance, excellent tolerance to polarization issues concerning the design and operation of multimode interdesign and behavior issues concerning I. and wavelength variations, and relaxed fabrication requirements when compared to alternatives such as directional couplers, adiabatic X or Y-junctions, and diffractive star couplers. in elaborate optical circuits ;Section ~ 1 1 ) . we conclude by comparing the properties of MMI devices with those of more conventional routing and coupling devices.