People reacted to the September 11, 2001, terrorist attacks in a number of different ways. One reaction was to display the American flag on one's home, car, or person. The goal of this research was to understand the underlying motivations that led to this widespread behavior. Specifically, to what extent was post-9/11 flag-display behavior motivated by patriotism (love of country and in-group solidarity), nationalism (uncritical acceptance of national, state, and political authorities and out-group antipathy), or a combination of both? Results of a national survey (N= 605) provided much stronger support for the hypothesis that post-9/11 flag-display behavior was an expression of patriotism, not nationalism. Other results supported the notion that patriotism can exist without nationalism, even in the context of people's reactions to a terrorist attack.