This volume provides a systematic archival inventory and critical study of the Leone Caetani Archive (Archivio Leone Caetani—ALC), preserved at the Accademia Nazionale dei Lincei in Rome. Spanning over 13,000 documents collected over nearly three decades, the collection illuminates the multifaceted life and intellectual trajectory of Prince Leone Caetani (1869–1935), a pioneering Italian historian, orientalist, and politician. Beyond Caetani's seminal contributions to early Islamic historiography—such as the monumental Annali dell'Islam—the archive captures rich epistolary exchanges with premier international scholars, domestic political figures, and prominent cultural networks of the Belle Époque. The present work outlines the complex custodial history and original organization of the fund while introducing an updated, "reasoned" inventory arranged alphabetically for optimal consulting. Accompanied by analytical abstracts, detailed indexes, and bibliographic apparatuses, this study establishes the archive as an essential primary source for researching early 20th-century intellectual history, orientalist historiography, and Italian parliamentary politics.