This chapter reports concepts and realizations of integrated optical circuits, i.e., the concept of optical waves which are traveling through planar circuits and confined in one or two directions. Integrated optical components such as array waveguide gratings (AWGs) turned out to be a key enabler for modern broadband optical communications based on wavelength division multiplexing (WDM). The requirements on integrated optical circuits range from passive functions such as beam splitters and optical filters over slow and fast optical switches to active transmitters and receivers. The material systems include glasses, polymers, LiNbO3, silicon, and III-V semiconductors. Weakly guiding integrated optical circuits, the traditional approach exhibiting waveguiding properties close to those of standard single-mode fibers (SSMF), offer ease of fabrication through single-mode field diameters of several wavelengths and large radii of curvature. Such components are based on a low refractive index contrast between the waveguide and its environment. Modern approaches aim at increasingly compact circuits to meet the system-driven requirements, mainly cost and footprint. They are based on waveguides with high refractive index contrast or photonic crystal waveguides and submicron waveguide cross-sections, e.g., on silicon, which exhibit single-mode field diameters much smaller than the wavelength and thus extremely small radii of curvature.