Recently, optical sensors have received considerable attention due to their unique properties such as resistance to electromagnetic interference, high sensitivity, small sensing unit, safety in hazards or explosive environments, possibility of processing the signal at large distances, and the ability to work under high temperature, high voltage, and high pressure conditions. The applications of optical sensor are also improved in all domains. A photonic crystal technology is used in order to reduce the size of the sensor without decreasing the quality of the sensor. In this chapter, the overview of optical sensors, photonic crystals, types of photonic crystals, and the primary applications of the photonic crystals is discussed. Moreover, the reported optical sensor using photonic crystal for biomedical application is presented in detail.