Buildings frequently change over their lifespans as they are adapted to new needs and affected by damage and decay, yet our approaches to architectural history often fail to account for the material and cultural effects of interventions on existing structures or to pursue the critical questions they raise about temporality and urban environments. The book’s Introduction orients readers to diachronic approaches to architectural history, that is, beyond the moment of initial construction, oriented to the perspective of historical actors. In recognizing moments of architectural revision and rebuilding as inflection points, it stresses the importance of accounting for architectural fabrics composed of variously dated elements and of examining the ways that architectural change shapes audience perception of the site’s history and their own era’s relationship to it. Close examination of two exceptionally long-lasting structures, the Pantheon in Rome and the Hagia Sophia/Ayasofia in Constantinople/Istanbul present a compelling contrast to most modern forms of architectural restoration and illustrate central themes of the book. The chapter situates study of historical architecture within current approaches to cultural time and to material culture and places architectural change in dialogue with text-based approaches to Roman temporality.