The factors which determine image contrast in optical and electron micrographs are discussed in relation to a new metal shadow‐casting technique whereby the contrast of images is greatly increased by depositing obliquely a thin film of metal on the microscope preparations. Further advantages of the use of shadow‐casting are described, and an estimate is made of the lower limit of size of objects which should be observable by shadow‐casting. Examples are given and illustrations are shown of the applications of this technique to the electron micrography of particles of macromolecular dimensions, of replicas of such particles, and of surface replicas prepared in several ways.