Contemporary Internet banks and other systems for money management are typically designed and evaluated on their usability; a system that is easy to use is considered a good system. However, insights from behavioral finance show many ways in which users of Internet banks are likely to be misled by e.g. certain ways of displaying data, and make the wrong decisions. This paper is a call for researchers and designers of money management systems to learn principles of finance before designing systems that will influence their users’ decisions.