Purchasing power is of particular concern to French citizens and forms part of the political debate, triggering recurrent controversies, partly due to the lack of a common definition of this term. It is easy to agree on the fact that purchasing power must reflect the simultaneous evolution of incomes and prices: if a household’s incomes increase faster than prices, it will gain purchasing power and can consume or save more; otherwise, it will lose purchasing power and must reduce its consumption or savings. The difficulty resides in giving a precise meaning to these different terms. The national accounts use definitions that have the advantage of being consistent with each other and internationally harmonised. However, they lead to evaluations that are often perceived as overly optimistic, even when care is taken to differentiate clearly between the overall purchasing power of all households and purchasing power per consumption unit, which is the most relevant concept from an individual point of view. Several factors can contribute to this discrepancy between measurement and perception. First, there is an inherent limitation to reasoning in terms of the average value. Even if there were complete agreement over the measurement conventions, there would still be a significant proportion of households whose purchasing power varied more slowly than this average rate. There are also perception biases in price measurement, which were documented in detail during the changeover to the euro. In addition, certain misunderstandings need to be clarified about some of the conventions used to measure income or its different uses: the inclusion of “imputed” income or the treatment of loan repayments, etc. Finally, the discrepancy may stem from the fact that households reason in terms of discretionary income, considering certain expenditure to be “unavoidable” and, in practice, deducting it – at least in the short term – from everyday consumer spending trade-offs. Further information about all these subjects can be provided by indicators other than the measurement of average purchasing power.