Presents theoretical calculations and experimental observations on electron scattering by gases in the specimen chamber of a scanning electron microscope. The observed scattering is in good agreement with theoretical calculations based on elastic and inelastic electron scattering. For normal surface inspection work, good resolution is retained in back-scattered imaging when gas is deliberately introduced into the specimen chamber. Experimental and calculated beam profiles are discussed for 25 keV electrons in nitrogen gas. Guidelines for choosing operating conditions for higher than normal gas pressures in the specimen chamber are also presented. Micrographs are provided to illustrate the image clarity attainable under high electron scattering conditions.