Perceptual learning involves relatively long-lasting changes to an organism's perceptual system that improve its ability to respond to its enviornment. Four mechanisms of perceptual learning are discussed: attention weighting, imprinting, differentiation, and unitization. Bby attention weighting, perception becomes adapted to tasks and environments by increasing the attention paid to important dimensions and features. By imprinting, stimuli that were once indistinguishable become psychologically separated. By unitization, tasks that originally required detection of several parts come to be accomplished by detecting a single constructed unit representing a complex configuration. Research from cognitive psychology, psychophysics, neuroscience, expert/novice differences, development, computer science, and cross-cultural differences is descirbed that relates to these mechanisms. The locus, limits, and applications of perceptual learning are also discussed.