On-chip optoelectronics allows the integration of optoelectronic functions with microelectronics. Recent advances in silicon substrate fabrication (silicon-oninsulator (SOI)) and in heterostructure engineering (SiGe/Si) push this field to compact (chipsize) waveguide systems with high-speed response (50-GHz subsystems realized, potential with above 100 GHz). In this paper, the application and requirements, the future solutions, the components and the physical effects are discussed. A very high refractive index contrast of the waveguide Si-core/SiO2-cladding is responsible for the submicron line widths and strong bendings realized in chipsize waveguide lines and passive devices. The SiGe/Si heterostructure shifts the accessible wavelength into infrared up to telecommunication wavelengths 1.30-1.55 μm. Germanium, although also an indirect semiconductor as silicon, offers direct optical transitions which are only 140 meV above the dominant indirect one. This is the basic property for realizing high-speed devices for future above 10 GHz on-chip clocks and, eventually, a laser source monolithically integrated on the Si substrate.