A phenomenonological model of topographic contrast in secondary electron microscopy is presented. This model involves exponential attenuation of isotropically generated secondary electrons. The effect of primary beam diameter and the material dependent secondary electron attenuation length on secondary electron images is demonstrated by explicitly computing linescans of images of straight surface steps. These computed linescans are directly compared with those obtained from monatomic surface steps on Si(100) imaged at normal incidence in an ultrahigh‐vacuum scanning transmission electron microscope. An asymmetry in the experimental linescan cannot be fit by any combination of model parameters suggesting that this contrast is not simply due to surface topography. A simple explanation for the contrast reversal observed in secondary electron images of surface steps when the primary beam changes direction from the upstairs to the downstairs direction is presented. The possibility of determining secondary electron emission parameters and extracting surface chemical and electronic information using high spatial resolution secondary and Auger electron imaging is briefly discussed.