Metal-organic frameworks (MOFs) are porous crystalline materials with tunable physical and chemical properties. This tuneability holds great promise for a wide range of applications in energy and environmental technologies, including water harvesting, gas storage and separation, catalysis, and sensing. MOFs are constructed from metal clusters (nodes) connected by organic linkers in a very versatile manner resulting in a great variety of MOF architectures. Nodes and linkers are typically chosen so that the resulting MOF architecture is adapted to a target application, for example, in terms of pore diameter, aperture and, connectivity. However, while this is conceptually straightforward, in practice, the synthesis of a new MOF is usually not an easy task and large-scale, reproducible production is challenging for most MOFs. As an alternative to designing entirely new MOF materials, post-processing techniques have been explored, whereby the properties of MOFs with well-established synthesis protocols are tailored by means of post-synthetic modifications. One such method involves the impregnation or infiltration of MOF powders with ionic liquids (ILs). ILs are salts comprising (most often large organic) cations and anions; they are usually liquid at temperatures below 100 °C. Comparable to MOF chemistry, there is a great variety of tunable ILs offering an extensive array of chemical and physical properties. In particular, their low vapor pressures make them more resistant to evaporation and combustion when compared to conventional organic solvents. Incorporation of ILs into MOF pores triggers specific interactions between the IL's ion constituents and the MOF's linker or nodes. In turn, such interactions -which are mostly electrostatic in nature -can alter the metal-linker interactions within the MOF. For example, redistribution of the electron density over the organic linkers or metal sites can induce catalytic activity, or provide new adsorption sites for guest molecules. IL-MOF composites have shown outstanding performance in various energy-related applications, for example, in gas adsorption, gas separation, heterogeneous catalysis, ion conduction, or glass-like processing ability. The vast majority of energy conversion processes result in the generation of thermal energy.