The matter of miracles offers a bracing and thoroughly original rethink of baroque relics, reliquaries, metals and materiality, through investigation of the miracle of the Neapolitan saint San Gennaro's liquefying blood in relation to art, philosophy, architecture and the city. Focused on the richly adorned baroque Treasury Chapel of San Gennaro, this study embraces sanctity and salvation in the material analogies at work among city, saint, volcano and blood to question the cultural impact of Spanish colonialism within Europe in the city of Naples. It examines the matter of the baroque miracle as transformational through a rigorous engagement with natural history, telluric philosophy, new materialism, theory and philosophy. Bronze and silver, architecture and sculpture are subjected to energetic interpretations, which give a vitally new approach to baroque sanctity, in which the city is seen as an event in the history of holiness. Bristling with new archival materials and historical insights, this study lifts the baroque from its previous marginalisation, to engage fiercely with materiality as potentiality and thus with art and architecture as potentially transformative. The matter of miracles will particular appeal to students and scholars of urban studies, art and architectural history and theory.