The wave of upbeat stories on the developing world’s emerging middle class has sparked a debate on how social class in general and the middle class in particular can be defined and empirically captured. This chapter provides a structured overview of a wide array of definitions that have been proposed in the economics literature, and assesses their strengths and limitations with an application to South Africa. Arguing that social class is insufficiently understood in terms of the current standard of living alone, it presents a conceptual framework that takes the unequal distribution of chances of social mobility explicitly into consideration when defining class categories. It compares the proposed approach to those that have been suggested in the previous literature, investigates the sources of upward and downward mobility, and illustrates the main messages that can be learned from linking the demarcation of social strata to an in-depth analysis of mobility patterns.