In a much-discussed passage in the introduction to the Grundrisse, Marx argues that ‘the concrete is concrete because it is the concentration of many determinations, hence unity of the diverse. It appears in the process of thinking, therefore, as a process of concentration, as a result, not a point of departure’. This chapter discusses how this conception has informed fieldwork on class relations in rural South Africa, where households combine a multiplicity of income sources, including local and migrant wage labour, self-employment in petty commodity production (both agricultural and nonagricultural) and welfare payments by the state. In these contexts, class differentiation is constituted through complex relations and processes, involving many social forms, including income stratification, diversified livelihood strategies and social difference along lines of race, gender and generation. Households often combine a variety of class locations that also shift over time. Here, ‘class’ cannot simply be read off from indicators such as incomes, assets or employment status. A key focus is social relations, including relations between employers and employees, property owners and land users, migrants and households, men, women and youth, as well as between interest groups and the state. Data collection and analysis are mediated by key concepts and theories drawn from Marx, such as the production and appropriation of surplus value, accumulation and social reproduction. The chapter describes how this approach – in fact compatible with critical realism – informed research in four different fieldwork projects undertaken in South Africa and briefly reports their findings. It also discusses the challenges posed in attempting to move from simple abstractions to an adequate account of the concentrated complexities of class relations, which increasingly involve the ‘fragmentation’ of class identities.