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. A period of change marked the middle of the century, when less advanced topics were presented and problem-solving tasks were simplified. From the mid-1960s onward, however, the trend reversed, and 3 major changes occurred in primary school mathematics curricula over the next 4 decades: (a) expansion of topics and the number of pages devoted to each topic; (b) a shift of traditionally more advanced topics from higher to lower grades; and, (c) within arithmetic, an increase in the number, abstraction, and cognitive demand of problem-solving strategies. Implications of these findings are discussed in terms of the historical study of mathematics and curriculum in U.S. schools.