An apparatus for exciting and detecting NQR in a substance containing quadrupole nuclei responsive to the NQR phenomenon. The apparatus includes a coil 15 for irradiating an item that may contain a substance with RF waves to excite NQR in quadrupole nuclei within the substance and for receiving an NQR signal emitted in response thereto. The apparatus also includes a transmitter unit 10, a receiver unit 11, a sensing means and a computer 12 for processing the treated received signal to identify a said NQR signal therein.