Research shows that many people make poor economic decisions because they are financially illiterate. As we show here, financial ignorance can be expensive and even ruinous, for many. Our goal in this article is to provide an overview of existing work in this important field. We also provide new evidence from the data collected by the Programme for International Student Assessment (PISA) of the Organisation for Economic Co-operation and Development (OECD). The article concludes with a discussion of the implications of findings for policy.