Systems of units still fail to attract the philosophical attention they deserve, but this could change with the current reform of the International System of Units (SI). Most of the SI base units will henceforth be based on certain laws of nature and a choice of fundamental constants whose values will be frozen. The theoretical, experimental and institutional work required to implement the reform highlights the entanglement of scientific, technological and social features in scientific enterprise, while it also invites a philosophical inquiry that promises to overcome the tensions that have long obstructed science studies.
1. Introduction; 2. The origins of the Metre Convention, the SI and the development of modern metrology; 3. Justifying and motivating an SI for all people for all time; 4. Reforming the International System of Units: On our way to redefine the base units solely from fundamental constants and beyond; 5. Strategies for the definition of a system of units; 6. Relations between units and relations between quantities; 7. On the Conceptual Nature of the Physical Constants; 8. And how experiments Begin: The International Prototype Kilogram and the Planck Constant; 9. The SI and the Problem of Spatiotemporal Constancy