The basic principles and mechanisms of photonic sensors are discussed, including the use of light to sense and quantify associated physical, chemical, or biological changes in their surroundings. Sensor effect in photonic sensors takes place through interaction of light with matter: a phenomenon based on both principles of optics and quantum mechanics, often manifested through absorption, reflection, refraction, and scattering phenomena depending on the interacting materials. It explains how such interactions make them precise for sensing applications by describing how the interactions occur. The various kinds of photonic sensors highlighted in the chapter, including fiber-optic sensors, which due to their ability to send data over long distances with minimal loss, interferometric sensors due to high sensitivity to detect minute changes in phase, and imaging sensors due to their visual representation of physical phenomena. Each type has specific applications where they are most useful-from telecommunications for data transmissions to healthcare, where diagnostics may be diagnosed in real-time and environmental monitoring where the ability to detect pollutants and changes in the conditions within the ecosystem is vital. By discussing these fundamental principles and mechanisms, we hope to broadly explain the operation and importance of photonic sensors in driving measurement technology improvements; ultimately demonstrating the hugely important differences it has made in various sectors of industry.