This chapter discusses the principles of chromic materials as applied to textiles. The most important chromic phenomena – thermochromism, photochromism, ionochromism and electrochromism – are dealt with in individual sections, each providing a description of the physicochemical principles underlying the colour changes and a discussion of the molecular structures of the most important colorant classes. An overview of the main non-textile applications of chromic materials sets the scene for more detailed discussion of progress through ongoing research towards technical and smart textile applications. The chapter concludes with informed speculation on likely future trends in the development of products based on chromic textiles.