Patterns of intermarriage between persons who have varying levels of educational attainment are indicators of socioeconomic closure and affect the family backgrounds of children. This article documents trends in educational assortative mating throughout the twentieth century in the United States, using socioeconomic data on adults observed in several large cross section surveys collected between 1972 and 2010 and on their parents who married a generation earlier. Spousal resemblance on educational attainment was very high in the early twentieth century, declined to an all-time low for young couples in the early 1950s, and has increased steadily since then. These trends broadly parallel the compression and expansion of socioeconomic inequality in the United States over the twentieth century. Additionally, educationally similar parents are more likely to have offspring who themselves marry within their own educational level. If homogamy in the parent generation leads to homogamy in the offspring generation, this may reinforce the secular trend toward increased homogamy.