Chapter 1 presents in statistical terms the debates that continually re-erupt whenever the CPI comes under strong political pressure. It presents the differences between a Laspeyres, a Paasche and a Fisher Indice. Chapter 2 reviews the consumer price index over time, presenting 8 generations of the CPI. The chapter focuses on the Fordist period (1945-1970). This period is characterised by a multiplicity of political pressures and ended in a sort of paroxysm with the publication by the major Trade Union affiliated to the communist party (the “CGT”) of an alternative indicator (1972-1998). Chapter 3 analyses a major European turning point which, following the introduction of the Maastricht criteria and then the transition to the euro, transformed the political and economic context. With the introduction of the harmonised index of consumer prices (HICP) and the increased role played by Eurostat, the analysis describes some transformations in the measure itself : the new focus on European integration and the increased circulation of mainstream ideas, revitalised by the emergence of neoliberal ideas in the USA, led eventually to a questioning of the very principle of a measure computed close to the market. Chapter 4 analyses takes account of the question of product quality. It renews attention on the incommensurability of individual preferences and utilities and shows how qualities are statistically quantified. It stresses on the numerous conventions that lay behind such quantifications. Chapter 5 examines the conflicts around definition, in particular that between a price index based on a constant basket of goods and services (COGI) and a cost of living index (COLI), which is consubstantial with the long history of the price index. For historical reasons specific to the France of the 1970s, INSEE maintains, at least in its communications, a strict division between a consumer price index and a cost of living index, as is the case in the European Union. In fact, however, the format of the data, the techniques employed and the dominance of utilitarian ideas are all factors that prefigure the possible advent of a constant utility index. Chapter 6 aims to trace the reform of checkout data, with an emphasis on the shift it has produced in the boundaries between the public and private spheres in the process of constructing the CPI. The recent changes towards the use of checkout data constitute a modern expression of the reuse of private data by a public actor. Chapter 7 begs the question that runs through the whole book: is the CPI ‘under’ or ‘over’-estimated? What are the social forces to convince either of these statements?