The contribution discusses the usefulness of the Franciscean cadastre as a source for studying the history of water infrastructure and water supply on the example of the tax district of Sežana. The author’s central question concerns the accuracy of the data provided by the cadastre under examination, the purpose of which—i.e. marking and registering water infrastructure—was of a relative nature. Cadastre files and statements from the Franciscean cadastre have largely been preserved in the Trieste State Archives and are also accessible online. Apart from archival materials, the author also drew on her own observations in the field as well as information obtained from oral sources.