Less than five years into the great euro experiment, the president of the European Commission at the time judged Europe’s limit on public deficits to be "stupid". A more intriguing question, however, is whether numerical constraints on broad indicators of fiscal performance can contain politicians’ penchant for borrowing too much and at the wrong time. This column summarises lessons from recent research on fiscal rules, including the new generation of ‘smart’ rules that emerged around the Global Crisis. Rules can mitigate fiscal excesses if they strike a good balance between simplicity, flexibility, and enforceability.