Considered the most important French Renaissance church, Saint-Eustache in Paris has long remained an enigma. What new circumstances allowed its parishioners, long desirous of a new church, suddenly to begin buiding it 1532? Did Francis I play a role? Was the obscure Jean Delamarre possibly its architect? Could the ideas of the Italian theorist, Serlio, have affected his design? These and other key issues are resolved by the author in a sustained reading of all known evidence. The baffling formal complexity of the church is clarified through lucid analysis that employs hundreds of new photographs executed by the author. The building is studied within the context of sixteenth-century French architecture and its roots in antiquity, the Italian Renaissance, Romanesque and Gothic France, and the Flamboyant Style. Sankovitch’s work will serve as a standard for all those who desire to understand this mysterious building and its times. A bright, clear window revealing an unseen architecture, previously an invisible — or at best murky — episode in the history of art, it is a portal to all future research on the building, and a key to the architectural life of the period.