This grand, ambitious book is about a way of building that for centuries dominated the making of monumental architecture - yet now not only is it lost as practice, but knowledge of its very existence is also lost. In the pre-modern age in Europe, the architect built not merely with imagination, brick and mortar, but with time, using vast quantities of duration as the means to erect monumental buildings that otherwise would have been impossible to achieve. This was not mere medieval muddling-through but entailed a highly developed set of norms and effective practices. Virtually all the great cathedrals of France and the rest of Europe were built under and by this regime, here given the name 'Building-in-Time'. To understand it puts in an entirely new light the major works of pre-modern Italy, from the Pisa cathedral group to the cathedrals of Milan, Venice and Siena, and from the monuments of fourteenth-century Florence to the new St Peter's, the apotheosis of the practice. In telling this story, Marvin Trachtenberg rewrites the history of medieval and Renaissance architecture in Italy and recasts the turn to modernity in new terms, those of temporality and its role in architectural theory and practice. Recovering this lost element of the deep architectural past allows us also to see the present in a new way: that temporality is not any neutral or secondary factor in modern architectural culture, but an epistemic condition that silently affects all production and experience of the built environment.