Alan Gilchrist, one of the leading researchers in achromatic perception, reviews the history of the scientific development of lightness theory from the nineteenth century until the present and outlines and critiques all the main theories of lightness, laying out the strengths and weaknesses of each. Based on thirty years of research, Gilchrist presents his own argument that previous models of lightness perception are too good because they fail to capture the errors and illusions present in human perception. These errors may contain crucial clues in the sense that the overall pattern of errors is the signature of the human visual system.