Porous polymers have been evolving continuously since the introduction of foam rubber in 1929. Today, pore diameters ranging from sub‐nanometre to millimetre can be generated controllably. Cutting‐edge porous polymers are now being applied at the forefront of critical problems with societal and environmental impact including advanced systems for biomedicine, water purification, energy storage, and gas purification and storage. The commonly‐used pore generation approaches include macromolecular design, self‐assembly, phase separation, solid and liquid templating, sol‐gel formation, and foaming. In each, The Chemistry of Polymers, both the polymerization chemistry and the macromolecular structural chemistry, must be applied advantageously to generate the empty volume within the polymer and then fix it in place. This essay will traverse the various pore size scales, describing the chemistries involved and discussing their implications.