Behavioural Economics (BE) acknowledges that individuals often make choices that are not entirely rational. The UK Competition and Markets Authority and Financial Conduct Authority have recently highlighted the relevance of BE. This article explains the difference it makes to the economic analysis of competition and why it is seen as particularly relevant to financial product markets. BE is already being used to frame and test theories of harm. It also brings experimental techniques to the analytical toolkit. The current approach is illustrated with examples from recent and ongoing cases. Finally, the risks of over-intervention and unintended harm from inappropriate remedies are highlighted.