The apparent contrast of a central stimulus is affected by the presence of surrounding stimuli. For some stimulus conditions, the apparent contrast is suppressed and for other conditions the apparent contrast is enhanced. This report is intended to offer a coherent description of the stimulus factors that influence suppression and enhancement. Using a contrast-matching protocol, we measured the contrast dependence of center-surround interactions by systematically varying the suprathreshold contrasts of the central and surround gratings. Different spatial configurations of the surround stimuli were studied. Our results confirmed previous findings that (1) a surround stimulus could produce either contrast enhancement or contrast suppression depending on the balance of the central and surround contrasts; (2) suppression varied with the width of the surround stimulus and was strongly orientation-specific; and (3) enhancement was less sensitive to changes in surround configurations (in particular, enhancement did not depend on the colinearity of the central and surround gratings). Based on the experimental data, we developed a computational model to account for center-surround suppression and enhancement.