Contrast and resolution in electron micrographs from thin replica films are determined by the geometrical relationships between the directions of incidence of the condensing atom beam and the local surface normal, during film formation by evaporation in vacuo, and the direction of incidence of the electron beam, during subsequent exposure in the microscope. Replica films may be formed of any material suitable for vacuum evaporation. Metal atoms in general lend to stick where they strike, moving only short distances, 100 Å or less, to nucleating centers where they form small crystallites. Oxides such as silica and silicon monoxide, and also the semi-metal germanium, form amorphous films. A portion of the incident material, about 50% in the case of silica, migrates large distances, 5000 Å or more, before finally condensing; the remainder sticks where it first strikes the surface. The existence of a minimum perceptible mass thickness difference, about 0.7 μUg/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for 50 kv electrons, results in an optimum replica mass thickness of about 10 μg/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The resolution of the replica film is proportional to its linear thickness and hence is inversely proportional to its density. Micrographs of silica, chromium, gold-manganin, aluminum, aluminum-platinum-chromium and germanium replicas are shown. The importance of stereoscopic methods in interpretation of micrographs is discussed.