Investigated the flexibility of perceptual processes in response to task demands in 6 experiments that focused on the effect of visual similarity of stimulus letters. Overall results indicate that (a) there are accumulation processes in the identification of letters by the perceptual system; (b) speed-accuracy criteria may also be perceptual in nature; (c) there is not just 1 evidence accumulation process in the perceptual system, but many; and (d) the perceptual system can make highly specific adjustments to task demands. Implications for general theories of an adaptive perceptual system are discussed, and in particular, similarities are noted between the mechanisms of perceptual adjustment identified in the present studies and those identified in studies of set, speed stress, and stimulus probability.