The International System of Units (the SI) tries to find or construct something that does not change with time and place, since such constancy is the best possible ground for definitions of fundamental measurement units. This problem of constancy has received scant attention within the philosophy of science, but is the topic of this chapter. It starts by highlighting two inevitable kinds of circularities, one semantic the other epistemic. Then it briefly mentions dependencies between base unit definitions, and at length discusses when a base unit is partly defined by a so-called derived unit. If the logical structure of the unit definitions in the New SI should be made transparent, then the distinction between base and derived units has to be completely deleted and replaced by statements about dependence relations. When the logical structure of the New SI’s kilogram definition is clearly seen, serious doubts are cast upon the metrological adequacy of that definition.