A number of studies on mechanism of an illusory contour and related experiments are reviewed. The critical evaluation suggests that no theory may be adequate which assumes the only one process for perception of the illusory contour. Even the multi-process-theory is not adequate as far as it assumes the illusory contour as a mere result of other processes such as depth-processing and brigntness contrast. Rather, we suggest that the illusory contour itself influences depth-perception and brightness enhancement.