Between 350 and 850 Constantinople emerged as both the greatest city of the Mediterranean world and a monastic centre of unparalleled importance. Drawing upon a wide range of sources, including a rich body of hagiographical evidence, this study documents the historical relationship between the city and its monks during this crucial formative period. Monks and nuns played a key role from the beginning. In 350 their numbers were few, yet their impact on local politics and the church was significant. By 850 their presence was felt everywhere - from the world of the imperial court and church, to the local economy, elite culture, social services and popular piety. This dramatic rise in the influence of local monasticism was the result of its impressive numerical growth over time, and hard-won success in adapting the singular call of the monastic life to the challenges of the great medieval metropolis and imperial capital.