This is the first work to offer a specific account of the social, religious and linguistic shift from a largely Arabic-speaking Muslim island in 1060 to a largely ‘Latin’-speaking Christian one by around 1250. The Norman kingdom of Sicily has often been regarded as an exceptional model of political and religious tolerance in the pre-modern world. But this work assesses evidence for the catastrophic and irreversible effects that demographic change and socio-religious pressures brought to bear on the island’s Arabic-speaking, and pre-dominantly Muslim, communities. Also examined is the unique evidence provided by the vast multilingual registers of lands and men from the royal administration. These explain how a Christian ruling elite from mainland Europe came to impose their rule over a largely Muslim population. Besides which, they reveal much of the complex social structures within Sicilian society and many elements of the peculiars that appear to have been spoken. Drawing on a wide range of source material, the considerable, but telling, difficulties in distinguishing Sicilian Muslims from Arabic-speaking Christians and ‘Greeks’ are explained as well as how language and religion acted as measures of identity in the increasingly antagonistic atmosphere between the island’s diverse communities. Overall, this defining work marks a turning point in the reconstruction of our understanding of the social history of ‘Norman’ Sicily.