[Début du chapitre] One of the most important public policy debates in recent years has been the reform of accounting standards toward “fair value” accounting. Financial institutions, especially banks and insurance companies, have been at the forefront of this debate and have been the most vocal opponents of this reform. Judging from the intensity of the arguments and the controversy that this reform has generated, there is clearly much more at stake than what may appear to be esoteric measurement issues. We review the main strands of this debate, and describe a framework of analysis that can weigh up the arguments on both sides. Far from being an esoteric debate, issues of measurement have a far reaching influence on the behaviour of financial institutions, and determine to a large extent the efficiency of the price mechanism in guiding real decisions.