This book represents a first attempt to comprehensively discuss and investigate causes and potential implications of changing patterns of spouse pairing in Japan and to consider similarities and differences with patterns observed in the USA and other low-fertility Western societies. In this book, research on educational assortative mating in Japan is summarized and updated. This book contributes to research on the demography of contemporary Japan by overviewing theoretical and empirical linkages between marriage behavior and processes of social and economic stratification. It also extends the large body of research on assortative mating and stratification by incorporating insights from the understudied context of Japan. The authors draw upon multiple data sources – both survey and administrative data – to update and extend previous research on “who marries whom” in Japan. The wide range of consequences considered includes income inequality, the intergenerational transmission of advantage and disadvantage, marriage and fertility timing, lifelong singlehood, childlessness, and the family roles of husbands and wives. Throughout the manuscript, Japan is considered in comparative perspective by employing the large USA and international literatures on assortative mating.