The chapter by Matthijs Kalmijn explores the phenomena of endogamy and homogamy, focusing on the causes, patterns, and trends of marriage within social groups. Kalmijn reviews various hypotheses that explain these phenomena, including individual preferences for certain characteristics in a spouse, the influence of "third parties" (such as family, church, and state), and the constraints of the marriage market. He then summarizes empirical research, addressing questions about the extent of endogamy and homogamy, changes over time, related factors, and the coincidence of different dimensions of partner choice. The chapter discusses the strengths and weaknesses of past research, noting the descriptive richness and multifaceted theoretical perspective of sociological studies, while also highlighting methodological issues such as lack of standardization and integration of empirical and theoretical work. The empirical section focuses on Western societies and examines patterns and trends in race/ethnicity, religion, and socioeconomic status, providing insights into how these factors influence intermarriage and homogamy.The chapter by Matthijs Kalmijn explores the phenomena of endogamy and homogamy, focusing on the causes, patterns, and trends of marriage within social groups. Kalmijn reviews various hypotheses that explain these phenomena, including individual preferences for certain characteristics in a spouse, the influence of "third parties" (such as family, church, and state), and the constraints of the marriage market. He then summarizes empirical research, addressing questions about the extent of endogamy and homogamy, changes over time, related factors, and the coincidence of different dimensions of partner choice. The chapter discusses the strengths and weaknesses of past research, noting the descriptive richness and multifaceted theoretical perspective of sociological studies, while also highlighting methodological issues such as lack of standardization and integration of empirical and theoretical work. The empirical section focuses on Western societies and examines patterns and trends in race/ethnicity, religion, and socioeconomic status, providing insights into how these factors influence intermarriage and homogamy.