It took until the first half of the 20th century for architects’ ideas to mature, in conjunction with the new materials of steel, reinforced concrete, and electric light, into the distinctive imagery now recognized as modern architecture. But that imagery was only the outward sign of new ways of organizing structure, space, and surface. The Conclusion clarifies that, for much of the 20th century, modern architecture stood for the place of the future—as related to the past—in the present. But the associations of those ideas about future, present, and past always remained complex, changing, and contested. For all its global effects, modernity was never a unified phenomenon.