NuclearNeutron detectors Power harnesses the energy generated through nuclear fission, and this clean energy provides a viable and cleaner alternative to fossil-fuel-based power plants. This helps in the decarbonization of the environment. At the heart of a Nuclear Power Plant is a nuclear reactor engineered to carry out a controlled nuclear chain reaction. The heat generated is thus converted to electrical power. For the safe and proper functioning of a nuclear reactor, Nuclear reactor instrumentation is a sine qua non. The term reactor instrumentation means all the systems deployed in any nuclear reactor to ensure control and safety during all stages of operation and otherwise. That is to say, the “state” of the reactor needs to be controlled and monitored during fuel loading, first approach to criticality (FAC), power operation, changes in power as per demand, shutting down of an operating reactor, at shutdown, during refueling, and also during any other situation. This being so, the present chapter attempts to make an overview of nuclear instrumentation in so far as the above points are concerned.