Differential interference contrast (DIC) was introduced to microscopy by George Nomarski over four decades ago and since the late 1960s has became increasingly popular. Today, Nomarski's DIC system is part of the basic equipment of advanced microscopes manufactured by all the leading firms in Europe, Japan, and USA. Its principle, specific properties (3-D appearing images, optical sectioning, directional sensitivity), and applications are discussed. Modulation contrast microscopy is also mentioned, and a short biography of G. Nomarski is also given.