This book succinctly traces the history of the metric system from early modern proposals of decimal measures, to the birth of the system in Revolutionary France, through its formal international adoption under the supervision of an international General Committee of Weights and Measures (CGPM), to its later expansion into the International System of Units (SI), currently formulated entirely in terms of physical constants. The wide range of human activities that employ weights and measures, from practical commerce to esoteric science, influenced both the development and the diffusion of the metric system. The roles of constants of nature in the formulation of the 18th-century metric system and in the 21st-century reformulation of the SI are described. Finally, the status of the system in the United States, the last major holdout against its everyday use, is also discussed.