 A lot of philosophical attention has been paid to how the meaning of statements concerning lengths and times changes in the context of special relativity. For example, we have learned from Stein (1968) that a statement such as “the length of the table is 2m,” while perfectly meaningful in the context of Newtonian physics, is either elliptical at best or meaningless at worst in the context of special relativity. However, comparatively little philosophical attention has been paid to how special relativity brings about corresponding changes to the third fundamental dimension in basic physics: mass. Yet, assertions concerning mass, and other dynamical concepts in mechanics into which it figures centrally, notably momentum and kinetic energy, also have their meanings changed in special relativity. 