This chapter is an introduction to the fundamentals of colour description, measurement, evaluation and appearance. Knowledge of theories and current practices related to these topics is paramount in comprehending how colour is recorded, reproduced and managed in imaging media, subjects that will be introduced in later chapters. Colour is evaluated in different ways. Physicists describe variations in the intensities of wavelengths of visible radiation. Colori-metrists describe the amounts of reference primaries which, when additively mixed, match a particular reference light. Sensory scientists describe an observer's response as a result of stimulating their visual sensory mechanism. There is no single meaning or definition attached to the word 'colour'. Although the word is understood by everyone, if we are asked to define colour, the answer is not so obvious. Further, if asked to describe a particular colour, for instance 'blue', it is difficult to do so without using an example, such as 'blue is the colour of the sea'.