Calculations have been performed on the contrast available in thick specimens. Ten modes of operation have been considered, six in the conventional electron microscope (CEM) and four in the scanning transmission electron microscope (STEM). Electrons passing through the specimen fall into four categories, elastically scattered, inelastically scattered, unscattered, and those scattered both elastically and inelastically. For the various operation modes these groups are taken in combinations to form practical signal intensities. In the calculation of practical intensities, the angular distribution of plurally scattered electrons is considered. The fraction of each scattered group passing through the microscope aperture has been calculated for various types of illumination. The practical intensities are used to calculate the contrast available in stained and unstained specimens. For very thick specimens bright field offers the best combination of contrast and signal intensity. Other contrast modes show a reversal in the sign of contrast for thicknesses close to one mean free path for elastic scattering. The equations for signal intensity and contrast are quite general, and apply to all values of voltage and instrumental resolution.