This talk will examine the changing social class composition of major western cities over the last 150 years, focusing particularly on the last 50 years. The changing social composition of cities has been a major social and political question since at least the mid-19th century. It was at this time that the advent of large scale industrialization and migration began to create a large new unskilled urban working class. Social commentators and politicians, from Booth to Mayhew to Engels, pointed to the social impacts on cities. Subsequently, the advent of Fordism in the early decades of the 20th century led to creation of a large industrial working class and another round of social changes. Most recently, deindustrialization, the growth of a post-industrial society, and the knowledge economy has led to what some see as the hollowing out of the class structure. This talk will consider the impact of these changes, which have been most intense in big cities, and the major intellectual debates over what is happening today, with evidence and examples from New York, London, Paris, Vancouver and other cities.