This chapter describes the principles of operation of the main types of neutron detector that are used in nondestructive assay. One section is devoted to gas detectors (3He and BF3) as well as boron-lined detectors, proton recoil detectors and fission chambers. A second section describes scintillation detectors. The description includes a discussion of light collection and signal acquisition. The measurement of neutron energy spectra is also described.