The technique of counting individual photons using single photon detectors is referred to as photon counting. Every time a photon is detected, a signal pulse is emitted by the single photon detectors. The counting efficiency of the device is determined by the quantum efficiency and electronic losses which are inherent in the system. The magnitude of pulses counted gives the total number of photons detected. Photodetectors are classified into Geiger counters, photomultipliers, scintillation counters, single photon avalanche diodes and superconducting nanowire single-photon detectors. Photon detection finds applications in many fields such as quantum information science, medical imaging, fiber optic communication, DNA sequencing, astrophysics and materials science. This book includes some of the vital pieces of work being conducted across the world, on various topics related to photon counting. It will also provide interesting topics for research which interested readers can take up. This book will serve as a valuable source of reference for graduate and post graduate students.