A review is presented of the present status of the technology of interference measurements of small changes of certain physical quantities. The capability limits of measurements with the aid of interference, imposed by diffraction and by noise in the radiation source and receiver, are analyzed. The most effective visual and photometric methods of measuring phase differences of interfering beams are described. The modulation method, in which a phase difference change of (10$^{-5}$--10$^{-6}$)·2$\pi$ can be detected, and the phase method used in automatic interferometers are considered in greater detail.