The Long-Term Trend (LTT) mathematics assessment of the National Assessment of Educational Progress (NAEP) used the same set of items from 1982 through 2004, including 20 items that were administered to 9-and 13-year-olds, 29 items that were administered to 13-and 17-year-olds, and 4 items that were administered at all three ages. This study used these items to identify areas of mathematics that had substantial gain from one age level to the next and to identify patterns of gain by age and how those patterns changed over time. Findings included the facts that older students usually did better on items than younger students, although item context and wording was often as important as mathematical knowledge in explaining differences in performance by age. LTT NAEP included a unique set of estimation items and analysis of performance on those items showed that students at all levels could identify an appropriate estimate only when numbers were small and the context for the estimate was familiar. The LTT data also indicated that addition and subtraction skills of 9-year-olds increased and the estimation skills of 13-and 17-year olds improved in some contexts from 1982 through 2004.