It has been said that the presence of colour requires three things: a source of illumination, an object to interact with the light which comes from this source and a human eye to observe the effect which results. In the absence of any one of these, it may be argued that colour does not exist. A treatment of the basic principles underlying the origin of colour thus requires a consideration of each of these three aspects, which brings together concepts arising from three natural science disciplines, chemistry, physics and biology. Although the principal aim of this textbook is to deal with the chemistry of dyes and pigments, a complete appreciation of the science of colour cannot be achieved without some knowledge of the fundamental principles of the physical and biological processes which ultimately give rise to our ability to observe colours. This chapter therefore presents an introduction to the physics of visible light and the way it interacts with materials, together with a brief description of the physiology of the eye and how it responds to stimulation by light, thus giving rise to the sensation of colour. In addition, the chapter contains a discussion of some of the fundamental chemical principles associated with coloured compounds, including a description of how dyes and pigments may be classified, followed by an overview of the ways in which the chemical structure of a molecule influences its colour properties. This section places special emphasis on azo colorants because of their particular importance in the...