Neutron flux is the most important signature in an operating reactor. The measurement of neutron flux is facilitated by detailed instrumentation for all the operating phases of the reactor. The regulation and protection in reactors is based on the in-core neutron monitoringNeutron monitoring. Neutron and gamma signatures are measured and interpreted by the operator for an informed decision. Dedicated instrumentation logics are implemented to guide the reactor to safe operation. The reactor control is affected so as to maintain the flux distribution over the entire core and power distribution in different control regions of the core. Based on the detailed spatial monitoring, the reactor is scrammed and brought to a safe shut down state. This chapter discusses the core monitoring aspects and its relation of neutronics in thermal reactors. Apart from neutronic instrumentation a brief on temperature, flow, and power measurements is also given. Delayed neutron monitoringNeutron monitoring is another important aspect with regard to safety of fuel and its operational limits. This chapter gives a brief overview of the principles of the various neutron monitoringNeutron monitoring aspects in thermal reactors. The role of simulators and approaches in advanced reactors for core monitoring is also briefly covered.