In this chapter, the authors examine the terminology of color and color appearance. This includes appearance attributes such as hue, chroma, lightness, brightness, and saturation, as well as viewing condition attributes such as surround and background. The authors explore several of the factors that influence color appearance and how they might cause basic tristimulus colorimetry to fail. They describe the meaning of different numerical scales. The authors discuss color appearance models, which attempt to predict these appearance attributes. Color is an attribute of visual perception. While seemingly straightforward at first glance, color appearance is governed by the extraordinarily complex human visual system. Visual psychophysics is a technique for examining the relationship between physical measurements of a stimulus with the perception of that stimulus. To gain an understanding of color, one of the foremost requirements is to have a basic understanding of the human visual system.