This chapter covers in individual sections the chemistry of the main colour change phenomena, namely photochromism, thermochromism, ionochromism, electrochromism, gasochromism, solvatochromism, vapochromism and mechanochromism, adding two not covered in the 1st edition; aggregation phenomena due to excitonic effects, and those due to plasmonic coupling in metal nanoparticles, as well as a miscellany of other chromisms, e.g. amorphochromism and biochromism. The applications of all of these chromic phenomena are wide, varied and growing in both leading edge technological and novelty outlets on consumer goods. For instance, both organic and inorganic photochromic materials are used not only in the well known ophthalmic applications but also as optical memories and switches. Thermochromism is commonly used to produce composite thermochromic pigments for a raft of applications where temperature change is visualised, while ionochromic materials have applications in optical sensors, carbonless copying paper and direct thermal printing. Electrochromism's applications include smart windows, rear view mirrors and displays. Colour changes due to plasmonic coupling of metal nanoparticles, especially gold and silver, are beginning to be exploited in various mainly biomedicinal analytical applications. Each section of this chapter describes in detail the applications of all of these important chromic phenomena.