AbstractWe study how to promote compliance with rules in every day situations. Having access to unique data on the universe of users of all public libraries in Barcelona, we test the effect of sending email messages with different contents. We find that users return their items earlier if asked to do so in a simple email. Emails reminding users of the penalties associated with late returns are more effective than emails with just a generic reminder. We find differential treatment effects by user types. The characteristics we analyze are previous compliance, gender, age, and nationality.