This comprehensive review focuses on the fundamental principles which influence the generation of important dyes and pigments, including the formation of dye molecules, the processing of insoluble colourants to produce useful crystal forms, the application of dyes to fibres and other media, and the environmental chemistry of dyes. In addition the mechanisms of diazotization, diazo coupling, dye photodegradation, dye genotoxicity and the liquid crystal formation of anionic dyes are covered.