This chapter defines the meaning of the concepts, ‘informal’ and ‘informalisation’, from the perspective of radical political economy. It explains what a class-oriented view of informal labour can look like. After exploring competing theoretical approaches to the informal economy, the book arrives at a definition of informality based upon observations of economic disadvantage, the role of state regulations and the different ‘forms of exploitation’ that generate and distribute economic value. The chapter also looks at the main methods used by Indian state institutions to measure and monitor changes in informal employment, focusing on the Economic Census.