Many charitable organizations believe it is worthwhile to solicit very small donations, particularly from young people, because these gifts form a habit of giving which leads to larger donations in the future. Indeed, there is some evidence of a positive correlation between giving when young and giving when old. However, such a correlation, by itself, does not constitute evidence of habit formation. Using data on alumni contributions to a university, we assess whether the correlation is due to habit formation—true state dependence—or to unobservable factors such as affinity to the school. We further examine whether habits form by the mere act of giving or based on the amount given. We implement an instrumental variables approach using the fact that performance of the school's athletic teams and solicitation by one's former roommates generate shocks to giving while young that are plausibly uncorrelated with giving when older. There is strong evidence of habit formation on the extensive margin, but not in the amount given. This finding has important implications for fundraising strategies, charities' accounting practices, and tax policy. (JEL D64, D91, D12)