This volume presents a selection of the key studies in which leading scholars since the beginning of the 20th century attempt to explain the phenomenally rapid expansion of the early Islamic state during the 7th century CE. The articles debate the causes for the conquest movement or expansion, the reasons for its success, the nature of the movement itself, the impact the expansion had on the countries affected by it, and the complex questions surrounding the sources on which historians have constructed their views of the expansion, and the reliability (or lack of it) of those sources. No articles devoted to the actual conquest of a given locality are included-hundreds exist-but a fairly extensive bibliography lists many of the more important contributions in this genre. The editor’s introduction addresses the phenomenon of the expansion and how scholars have approached and grappled with it.