Block polymers comprising covalently joined polymeric segments represent a class of nanostructured, multicomponent polymeric materials. Polymerization-induced microphase separation (PIMS) is an intriguing subset that allows for simultaneous nanostructuring during block polymer synthesis. In contrast to polymerization-induced self-assembly (PISA), useful for the spontaneous formation of block polymer micelles, PIMS is well suited to fabricating monolithic block polymer materials by turning a whole polymerization mixture into a nanostructured solid. With the in situ cross-linking feature, PIMS offers a facile route to nanostructured block polymer thermosets in combination with various polymerization techniques, from emulsion polymerization to 3D printing. This review aims to provide a comprehensive overview and practical guide on PIMS by covering its historical background and mechanistic aspects and also highlighting representative material classes and applicable polymerization techniques.