Aniline and azo dyes were the first of many new chemical substances synthesised in the nineteenth century from coal tar. Manufactured for use as textile dyes, they soon began to be added to food, becoming one of the first examples of laboratory-created, industrially manufactured chemicals to be consumed in unexpected ways. This book examines how chemists and physicians, politicians and campaigners, food and dye producers, retailers, and the public jostled to determine and arbitrate the use of these new substances in Britain, Germany, France, and the US. The dyes were among the earliest contested chemical additives in food and the battles surrounding their use offer insights into international struggles surrounding chemical, food and trade regulation. Chemists maneuvered themselves to become key players in assessing food quality amid cultural differences across Europe and in America that led to different regimes of defining and regulating food products. Strategies for testing these novel ingredients were devised by chemists from different backgrounds and with different objectives using a variety of analytical repertoires. Despite being brilliantly colored and increasingly ubiquitous, these novel substances remained almost impossible to detect for decades. Although created by chemists, the detection and evaluation of synthetic dyes in food represented a failure in analytical chemistry. While commerce, politicians, and the public all invoked chemists to represent their interests, the authority, technical ability and impartiality of the scientists were never sufficient to successfully arbitrate on the issue, leaving the situation still unresolved in the twenty-first century.