Porous liquids are a class of porous material that has recently caught the attention of researchers. First envisaged in 2007 by James and co-workers, they later became a reality in 2015 where the first porous liquids were realised in the laboratory. These liquids differ from traditional solvents by having intrinsic micropores, allowing these materials to retain permanent porosity that leads to increased gas solubility and, in some cases, gas selectivity; that is, properties that are difficult to achieve with conventional non-porous liquids. Porous liquids can be classified by the following categories: Type I porous liquids are discrete liquid hosts where the liquid component has shape-persistent molecular porosity; Type II porous liquids are prepared by dissolving discrete porous molecules, such as porous organic cages (POCs), in a solvent that is excluded from the pores; Type III porous liquids are dispersions of porous solid particles, such as metal organic frameworks (MOFs), in a fluid where the solvent is excluded from the pores; Type IV porous liquids are materials with extended connectivity in three dimensions that retains porosity in the liquid state. Since their conceptualisation, there have now been reports of all four types of porous liquids. While porous solids are applicable in a wide range of applications, including molecular separations and catalysis, liquids have potential differentiable advantages because they can flow and might also promote more rapid heat transfer and dissipation. By contrast, the porosity per unit volume for porous liquids is thus far much lower than for porous solids. As such, porous liquids might allow for new, different processes, rather than simply replacing porous solids.