A new urban dialect has emerged in multicultural areas of London, as in many other European cities with a recent history of large-scale immigration, but there is little evidence of this kind of linguistic innovation in similar areas of Paris. In this lecture I present examples of young people’s vernacular language in London and Paris, and suggest explanations for the different outcomes of language contact in the two cities. The explanations include language policies in the former colonies, educational policies on the integration of newly arrived immigrant children in school, the relationship between social class and space in the city, the relative salience of ethnicity amongst young people, and the portrayal of minority speakers on YouTube and other media. Most of these factors are reflexes of over-riding national ideologies that pervade past and present policies and practices. Doing comparative research of this kind highlights the relevance of these ideologies and shows that they can shape even what might seem to be the most straightforward decisions about research design.