  We discuss the experimental consequences of hypothetical time variations of
the fundamental constants. We emphasize that from a purely phenomenological
point of view, only dimensionless fundamental constants have significance. Two
classes of experiments are identified that give results that are essentially
independent of the values of all constants. Finally, we show that experiments
that are generally interpreted in terms of time variations of the dimensioned
gravitional constant $G$ are better interpreted as giving limits on the
variation of the dimensionless constant $\alpha_G=Gm_p^2/\hbar c$.
