The ‘inappropriate size-depth’ theories suggest that illusions are to be explained in terms of mistaken perceptual impressions of depth cues supposedly implicit within certain spatial patterns. Contrariwise, the distortions evidenced in these same patterns may be understood as depending upon the ‘contour-proximity’ principle without any reference being made to apparent depth as such. Six experiments are described and an attempt is made to establish which of these two alternative explanations accounts most consistently for the results. That parts of illusions might be interpreted as differing in apparent distance is demonstrated in the first two experiments. Doubts are raised over this type of explanation in the second two studies, however, and the final experiments suggest that depth features themselves are neither necessary, nor sufficient, conditions for the appearance of illusory distortion.