The perceived brightness of a gray patch depends on the surrounding context. For example, a medium-gray patch appears darker when placed on a bright background and brighter when placed on a dark background. Models to explain these effects are usually based on simple low-level mechanisms. A new set of brightness illusions cannot be explained by such models. In these illusions, the brightness percept is strongly influenced by the perceptual organization of the stimuli. Simple modifications of the stimuli that should have little effect on low-level mechanisms greatly alter the strength of the illusion. These effects may be ascribed to more complex mechanisms occurring later in the visual system.