We review previous efforts to recover surface shape from image irradiance in order to assess what can and cannot be accomplished. We consider the informational requirements and restrictions of these approaches. In dealing with the question of what surface parameters can be recovered locally from image shading, we show that, at most, shading determines relative surface curvature, i.e., the ratio of surface curvature measured in orthogonal image directions. The relationship between relative surface curvature and the second derivatives of image irradiance is independent of other scene parameters, but insufficient to determine surface shape. This result places in perspective the difficulty encountered in previous attempts to recover surface orientation from image shading.