The methods of resonant interferometry and holography are discussed, and their ranges of applicability are estimated. Resonant methods enjoy a high sensitivity and a high selectivity in comparison with ordinary interferometric and holographic methods. Their primary field of application is with dense plasmas, although in individual cases it is possible to determine atomic densities down to N/sub a/ = 10/sup 9/ cm/sup -3/ and below. For a plasma with N/sub a/ = 10/sup 18/-10/sup 19/ cm/sup -3/ the minimum detectable atomic density is about N/sub a/ = 10/sup 14/ cm/sup -3/. The specific requirements on light sources and methods for studying them are described. The capabilities of these methods are demonstrated for the cases of potassium, sodium, lithium, and hydrogen plasmas; the atomic and electron densities are determined, and the plasma dynamics is studied.