Microporous polymers (MPs) are studied for their intriguing chemistry and physics as well as their potential application in catalytic transformations, gas-separation processes, water purification and so on. Here, we critically review MPs with respect to the sustainability aspects of their synthesis as well as their applications that have sustainable character. Some MPs have been synthesized from monomers derived from biomass resources, but there is certainly a large potential for further developments. There are also opportunities to improve the sustainability of MP synthesis in terms of the use of solvents, catalysts, and related aspects. The applications of MPs in processes related to sustainability depend upon multiple properties. A rich and flexible chemistry is important to applications as catalysts for, among other useful reactions, the photoreduction of CO2 and selective oxidation. The (ultra) micropore volume of MPs are crucial in gas-separation applications such as CO2 capture, and the chemisorption of CO2 on MP-tethered alkylamines could offer a means to remove that gas from dilute mixtures. When it comes to the storage of H-2 and CH4 in MPs for onboard use in fuel cell or biogas cars, volumetric capacity is paramount, meaning that the density of the MPs must be considered. Finally, for use in separation and purifications from liquid mixtures (aqueous or hydrocarbon-based), crosslinked MPs are more limited than the solution-processable MPs that can be more easily processed into films and membranes.