How should democratic societies and the cities that propel them respond to increased social diversity' As the aftermath of 9/11 reminds us, human diversity is often cast in national and even global terms, but inevitably it is lived - some would say endured - locally. Surprisingly few studies compare cities on their capacity to manage social diversity or offer historical views of the bases for co-existence among groups. Most studies of this crucial theme that do offer comparative reach are limited to nation-states in the modern global order, group effects on urban inequality, or ethnic politics in contemporary cities. This paper analyzes three large, history-making and famously diverse cities that relied on quite distinct recipes (composite arrangements) to accommodate diversity: ancient Rome, medieval Cordoba, and contemporary Los Angeles. Comparisons across such huge spans of time and major culture shifts yield lessons obscured in current debates over race relations, multiculturalism, or the need for tolerance. Three of the most important lessons relate to the power of massive, integrative societal projects; the co-existence throughout history of separatism or cultural mosaic patterns alongside active cross-cultural exchange and fusion; and the need to bound pluralistic ideals within a strong, locally viable public order. In earlier periods of history, autocracy provided such order for cities and the civilizations they helped to define. learn more on SSRN