A silicon microring resonator can be used to magnify the interaction between light and matter and its output is very sensitive to its index of refractive change caused by the interactions, which in turn will shift the resonating wavelength and the coupling coefficient of the resonator. This chapter describes the basic structure of the microring sensors, its working principle, and its unique properties. Sensitivity, selectivity, dynamic range, and thermal stability are most important properties for microring optical sensor. The conventional silicon microring sensor is based on single microring, where only the single symmetric Lorentzian resonance lineshape was used to respond to the index change. The thermo-optical effect can make the microring resonance spectrum shifting, which typically changes the intensity output at operation wavelength to distort the sensing single. The label-free biosensing techniques have shown the potential to simplify clinical analyses. Dynamic range is defined as the ratio of the sensor's largest detectable wavelength shift to its smallest signal.