On-chip sensing is an important application of silicon photonics. In this talk, silicon-based passive optical devices for on-chip sensing systems will be presented in two parts, the sensors and the supporting devices. The first part highlights the research on sensitivity enhancement, athermalization, and cost reduction of silicon photonic sensors while the other part presents the needed passive functional devices for a sensing system, including grating couplers, power splitters, polarization beam splitters, polarization rotators, and strip-slot waveguide mode converters.