The principle of holographic interferometry is described and its importance for plasma diagnostics is indicated, particularly for the determination of electron concentration (Section 2). A review is given of different methods for increasing the sensitivity associated with penetration into the infrared region of the spectrum, and also with the use of multiple-pass, resonance, dispersive nonlinear and two-wavelength holographic interferometry. Section 3 discusses the experimental techniques--recording media and radiation sources--as well as the results of fringe shift measurements, stroboscopic holography and cineholography. Section 4 gives a review of work on investigation of various plasmas--laser-induced sparks, laser jets, neutral current layers, $z$- and $\theta$-pinches, flash lamps, CO$_2$ laser-induced plasmas, exploding conductors, plasmotrons, electric arcs and various other kinds of electric discharges in gases.