A content analysis of over 28,000 pages from 141 elementary school mathematics textbooks published between 1900 and 2000 shows that widely used mathematics textbooks have changed substantially. Textbooks from the early part of the century were typically narrow in content but presented substantial amounts of advanced arithmetic and also asked students simultaneously to engage with material in effortful and conceptual ways. Implications of these findings are discussed in terms of the historical study of mathematics and curriculum in U.S. schools. (Contains 11 figures and 14 footnotes.)