Polymers below their glass transition temperature (Tg) are non-equilibrium glasses because excess free volume “frozen” between kinetically-restricted polymer chains slowly relaxes over time towards equilibrium via local, segmental chain motion. This process, known as physical aging, is observed through time-dependent decreases in a polymer's specific volume, enthalpy, etc. This article focuses on the history of glassy polymers in membrane separation applications. Open questions regarding the influence of thickness (e.g., membrane geometry) and temperature on physical aging in glassy polymers are highlighted. Glassy polymers have non-equilibrium excess free volume. Consequently, they typically have time- and history-dependent properties. For example, membrane gas permeance often gradually decreases with time. Display omitted •A polymer below its glass transition temperature has non-equilibrium excess volume.•Glassy polymers have additional sorption capacity relative to rubbery polymers.•Non-equilibrium excess volume affects polymer properties (e.g., transport, optical).•The thickness- and temperature-dependences of physical aging remain open questions.•Glassy polymers are frequently applied in gas separation membrane applications.