Research into extended porous materials such as metal-organic frameworks (MOFs) and porous organic frameworks (POFs), as well as the analogous metal-organic polyhedra (MOPs) and porous organic cages (POCs), has blossomed over the last decade. Given their chemical and structural variability and notable porosity, MOFs have been proposed as adsorbents for industrial gas separations and also as promising filler components for high-performance mixed-matrix membranes (MMMs). Research in this area has focused on enhancing the chemical compatibility of the MOF and polymer phases by judiciously functionalizing the organic linkers of the MOF, modifying the MOF surface chemistry, and, more recently, exploring how particle size, morphology, and distribution enhance separation performance. Other filler materials, including POFs, MOPs, and POCs, are also being explored as additives for MMMs and have shown remarkable anti-aging performance and excellent chemical compatibility with commercially available polymers. This Review briefly outlines the state-of-the-art in MOF-MMM fabrication, and the more recent use of POFs and molecular additives.