This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book focuses on the importance of a group of arguments related to the realization of the base units: the new definitions promote the universality, freedom and equality of access to primary realizations of the base units. It demonstrates how the redefinition rests on a link between mass and proper time that involves relativistic quantum theory, which can be investigated using atomic interferometry. The book addresses the crucial question as to whether such a link is significant at the macroscopic level of the kilogram. It explores philosophy of science to pay more attention to the issue of the coherence of systems of units. The book examines to what extent the distinction between base and derived units can really be dispensed with in a system of coherent units.