A photonic sensor is a device that is made of a p-n junction, which converts photons into current. Some examples of photosensors are photodiodes and phototransistors. Polarization, photoelectric effect, photochemical changes and weak interaction effects are the mechanisms fundamental to the working of photonic sensors. A number of performance characteristics are important for the characterization of photonic sensors. These include response time, spectral response, detectivity, responsivity and quantum efficiency, among others. Depending on the mechanisms or the technology involved in the detection of light, photosensors can be classified into semiconductor, photoelectric, photovoltaic, photochemical and thermal photosensors. Some photoelectric photosensors are phototubes and photomultiplier tubes. Active-pixel sensors, charge-coupled devices, etc. are some of the semiconductor photosensors. This book includes some of the vital pieces of work being conducted across the world, on various topics related to photonic sensors. The various advancements in this field are glanced at and their applications as well as ramifications are looked at in detail in the book. It will prove to be immensely beneficial to students and researchers in this field.