This project examines the physical evidence of one medieval building to probe the circulation of architectural practice in the Mediterranean during a period of expanding commerce and communications. Molfetta Cathedral embodies a discontinuous building process and a heterogeneous repertoire, combining local and foreign structural systems and decorative techniques. In contrast to the emphasis that previous studies have placed on form and style, I use archaeological methods to unravel this work site as an intersection of diverse and shifting practical expertise in the fields of stone-cutting, assembly, engineering, and construction process. This allows me to draw more precise comparisons than before, and, on the basis of these comparisons, analyze the circulation of architectural practice on a map using a geographic information system (GIS). The cathedral emerges as a junction of pan-Mediterranean, rather than purely local, systems of church building.