Reinforced concrete lends itself to a structural form called the cantilever, where a slab or beam is hung outwards from one side, suspended without columns, counter-weighted by a mass of structure behind. ‘Reinforced concrete’ explains how concrete structures came to represent radically opposing ideas of high capitalism and communism in the 20th century. It explores how concrete—paradoxically liquid and solid, formless and formed, natural and human-made—became associated with attempts to rethink social order and mechanized production. The work of Le Corbusier is described, with the development of Garden Cities, megastructures, and brutalism. Concrete encouraged the reimagination of modern architecture, but its peculiarities also exposed modern architecture to question.