This lecture covers neutron detection in nuclear reactors, including the fission chain reaction, neutron flux distribution, and the challenges of in-core neutron detection. It introduces the MiMi neutron detectors, their design, and applications in high-resolution experiments. The presentation also discusses the SAFFRON project, a 3D full-core mapping system using MiMi detectors in CROCUS. Experimental results on the performance of MiMi detectors and their comparison with other detectors are presented, along with the need for high-resolution neutronics measurements. The lecture concludes with future research directions and the importance of validating high-fidelity neutronics codes.