Silicon-on-Insulator (SOI) is a very interesting material system for highly integrated photonic circuits. The high refractive index contrast allows photonic waveguides and waveguide components with submicron dimensions to guide, bend and control light on a very small scale so that various functions can be integrated on a chip. Moreover, SOI offers a flexible platform for integration with surface plasmon based components which in turn allows for even higher levels of miniaturization. Key property of both waveguide types is the mode distribution of the guided modes: a high portion of the light is concentrated outside of the core material, thus making them suitable for sensitive detection of environmental changes. We illustrate chemical and label-free molecular biosensing with SOI microring resonator components. In these microring resonator sensors, the shift of the resonance wavelength is measured. A ring of radius 5 micron is capable of detecting specific biomolecular interaction between the high affinity protein couple avidin/biotin down to a few ng/ml avidin concentration. We describe the integration of surface plasmon waveguides with SOI waveguides and discuss the principle of a highly sensitive and compact surface plasmon interferometric sensor suitable for biosensing. The device is two orders of magnitude smaller than current integrated SPR sensors, and has a highly customizable behavior. We obtain a theoretical limit of detection of 10-6 RIU for a component of length 10 microns. We address material issues and transduction principles for these types of sensors. Besides in chemical sensors, the SOI microring resonators can also be used in physical sensors. We demonstrate a strain sensor in which the shift of the resonance wavelength is caused by mechanical strain. We have experimentally characterized the strain sensors by performing a bending test© (2007) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.