In complex scenes the same gray material appears the same in different places in the scene. In simple displays, grays vary in lightness with surround. By definition “contrast” is the name of the mechanism that makes grays look darker in a white surround than in a black surround. It is generally believed that the white surround stimulates inhibition of the center, making that gray look darker. The black surround does not generate inhibition and hence the gray appears lighter.Assimilation is the name of the mechanism with the opposite effect. Grays with adjacent white no longer look darker than the same gray with adjacent black. Examples are Benary's Cross, White's Effect, Checkerboard and Dungeon Illusions. These effects have been used to suggest a top-down analysis of the scene, implying mechanisms based on the recognition of illumination, objects or junctions.Recent experiments demonstrate that contrast is much more complex than inhibition by average luminance in the surround. Displays with a square, gray central element and 8 square surround elements demonstrate significant sensitivity to the placement of white and black surround elements. Equalaverage surrounds do not give equal gray appearances. Other experiments show that periodic assimilation effects are sensitive to average luminance over very-large-receptive fields. All of the above assimilation effects have gray center lightnesses that correlate with large-receptive-field averages. Contrast is the result of complex spatial interactions, while assimilation is due to large receptive field averages. Experiments studying the transition from contrast to assimilation are described.