Penetrant-induced plasticization in microporous polymer membranes has hindered the industrial use of many polymers for gas separations. With the development of microporous polymers, new structural features and property sets are now accessible under controlled conditions, but these properties can often deteriorate due to plasticization. Therefore, understanding the origins of plasticization in microporous polymers and developing strategies to mitigate this effect are crucial for advancing this area of research. This review provides an integrative discussion of seminal plasticization theory and gas transport models, comparing these theories and models to an extensive database of plasticization characteristics of microporous polymers. Correlations between specific polymer properties and plasticization behavior are presented, including analyses of plasticization pressures from pure-gas and mixed-gas permeation tests for pure polymers and composite films. An evaluation of common and current state-of-the-art strategies to mitigate plasticization is provided, along with suggestions for future directions of fundamental and applied research on the topic. Glassy polymers are viscous metastable solids that undergo structural reorganization over time, leading to decreased permeability. Plasticization refers to the increase in polymer chain mobility in the presence of condensable diluents, which can lower the glass transition temperature of the polymer. Plasticization often results in increased permeability but decreased selectivity. The plasticization pressure is the minimum value in permeability, indicating the point at which increased diffusivity balances decreased sorption. Plasticization is more severe for larger gases and can significantly affect gas selectivity in industrial applications. The plasticization of polymer membranes is influenced by the properties of the penetrant, such as its condensability and polarizability. The Flory–Huggins model and other thermodynamic lattice models are useful for quantifying interactions between polymers and penetrants. The dual-mode sorption (DMS) model is widely used to describe sorption in glassy polymers, where the pressure dependence of penetrant concentration is the sum of sorption into Henry and Langmuir modes. The DMS model has been extended to account for plasticization effects through the incorporation of a swelling parameter that describes the change in polymer density as a function of penetrant pressure. The effect of penetrant-induced plasticization on gas transport is significant, as it can lead to increased permeability and decreased selectivity. High-pressure pure-gas permeation tests are commonly used to evaluate plasticization, but these tests are limited in their ability to predict property sets under realistic conditions. Mixed-gas experiments provide a more comprehensive evaluation of plasticization, as they can track emergent phenomena such as competitive sorption effects. Permeation and sorption hysteresis curves are also used to examine the effect of plasticization and conditioning after high-pressure tests. Polymer chain cooperativity and the glass transition temperature (Tg) are critical factors in plasticization. Polymer chain cooperativity describes the collective motion of polymer segments as theyPenetrant-induced plasticization in microporous polymer membranes has hindered the industrial use of many polymers for gas separations. With the development of microporous polymers, new structural features and property sets are now accessible under controlled conditions, but these properties can often deteriorate due to plasticization. Therefore, understanding the origins of plasticization in microporous polymers and developing strategies to mitigate this effect are crucial for advancing this area of research. This review provides an integrative discussion of seminal plasticization theory and gas transport models, comparing these theories and models to an extensive database of plasticization characteristics of microporous polymers. Correlations between specific polymer properties and plasticization behavior are presented, including analyses of plasticization pressures from pure-gas and mixed-gas permeation tests for pure polymers and composite films. An evaluation of common and current state-of-the-art strategies to mitigate plasticization is provided, along with suggestions for future directions of fundamental and applied research on the topic. Glassy polymers are viscous metastable solids that undergo structural reorganization over time, leading to decreased permeability. Plasticization refers to the increase in polymer chain mobility in the presence of condensable diluents, which can lower the glass transition temperature of the polymer. Plasticization often results in increased permeability but decreased selectivity. The plasticization pressure is the minimum value in permeability, indicating the point at which increased diffusivity balances decreased sorption. Plasticization is more severe for larger gases and can significantly affect gas selectivity in industrial applications. The plasticization of polymer membranes is influenced by the properties of the penetrant, such as its condensability and polarizability. The Flory–Huggins model and other thermodynamic lattice models are useful for quantifying interactions between polymers and penetrants. The dual-mode sorption (DMS) model is widely used to describe sorption in glassy polymers, where the pressure dependence of penetrant concentration is the sum of sorption into Henry and Langmuir modes. The DMS model has been extended to account for plasticization effects through the incorporation of a swelling parameter that describes the change in polymer density as a function of penetrant pressure. The effect of penetrant-induced plasticization on gas transport is significant, as it can lead to increased permeability and decreased selectivity. High-pressure pure-gas permeation tests are commonly used to evaluate plasticization, but these tests are limited in their ability to predict property sets under realistic conditions. Mixed-gas experiments provide a more comprehensive evaluation of plasticization, as they can track emergent phenomena such as competitive sorption effects. Permeation and sorption hysteresis curves are also used to examine the effect of plasticization and conditioning after high-pressure tests. Polymer chain cooperativity and the glass transition temperature (Tg) are critical factors in plasticization. Polymer chain cooperativity describes the collective motion of polymer segments as they