Three conclusions about collective memory are developed from data obtained before and after the September 11, 2001 terrorist attack on the United States. First, generational effects, as predicted by the critical years hypothesis, show that a dramatic new national or world event can have an impact on young cohorts that leads to a lasting collective memory. Second, for the September 11, 2001 attack, that effect was visible almost immediately after the event. Third, once a collective memory is established, it is retained by the affected cohorts as they age, as we show for the Vietnam War. We conceptualize the population as consisting of successive cohorts, each of which carries collective memories arising from major events that occurred during its critical years.