This study investigates middle graders' understanding of significance in United States history through open-ended interviews with 48 students in grades 5-8. Students were presented with a set of captioned historical pictures and asked to choose the one important enough to include on a timeline of the last five hundred years, and to explain their choices. Students' choices focused primarily on the origin and development of the political and social structure of the United States, and their explanations pointed to steadily expanding rights and opportunities as a central theme in U.S. history. At the same time, students had difficulty incorporating some historical patterns and events into their image of progress, and their discussion of these issues indicated a familiarity with a"vernacular" view of history separate from "official" views justifying the contemporary social structure. This study suggests that students need experience with the complexities of the past within a context that provides some framework for making critical sense of both legitimating stories and alternative, vernacular histories. The paper suggests one way to help students build such a framework is by letting them grapple with exactly those issues--race, gender, and class, for example--that have not been so thoroughly digested that their meaning is firmly established in the historical canon. (Contains 36 references.) (Author/EH)