The models used in colorimetry are remarkably simple. Coloris encoded with respect to a three-dimensional, linear modelthat can be measured with appropriately designed instruments. However, the truth about the perception of color is much more complex. Color matching as described above only works for individual colors viewed under simple, controlled circumstances. Its definition is based on the perception of colored light. In the real world, we see complex arrangements of colors. These colors interact in ways that defy the simple model presented by colorimetry. This chapter will describe the key concepts and models used in the study of color appearance. These influence the application of digital color, both for image and information display.