Metastable states are not in true equilibrium and so cannot be directly treated by thermodynamics and statistical thermodynamics. To circumvent this difficulty one can specify an equivalent equilibrated state, equivalent in the sense that the observed macroscopic features are indistinguishable from those of the metastable state, and equilibrated because of the imposition of auxiliary constraints which totally block the very slow relaxations that cannot be completed during the time of the experiment. This procedure permits one to treat metastable systems consistently within a completely time independent and causal thermodynamic framework. It also gives a consistent description of the entropy of glassy and similar random metastable systems in which the entropy vanishes as T-->0 K, and it explains the apparent residual entropy at 0 K obtained in most conventional analyses based upon experiments carried out over irreversible paths (but not always recognized as such).