The physical properties of polymers are very much dictated by where the operating temperature lies with respect to the transition temperature between glassy and rubbery states. The precise identification of this glass transition temperature, Tg, is critical in assessing the feasibility of a polymer for a given application. In this book, the behavior of polymers near their Tg and the capability of predicting Tg using theoretical and empirical models is assessed. While all polymers undergo structural relaxation at various temperatures both nearly above and below Tg, practical assessment of a single consistent Tg is successfully performed through consideration of only immediate thermal history and thermodynamic properties. The determination of Tg for a wide variety of polymers of theoretically infinite chain length has been found to be accurately performed through the use of novel quantitative structure-property relationship (QSPR) models. The supplementation of such values to configurational entropy and molecular weight relational models can be used for estimation of Tg for shorter chain length polymers within the same homologous series, but only to a specific lower bound.