Partner choice represents a crucial stage in the process of family formation, and plays a key role in the reproduction of socio-economic and cultural inequality, in the development of kinship bonds, and in the enactment of gender. Given its intricate links with broader societal shifts, partner choice represents a particularly exciting area of study, not just for family sociology and demography, but for sociology in general. The term 'partner choice' is used interchangeably with the terms 'mate choice' and choice of a 'romantic partner' (Lewis 2016;Schwartz 2013). The adjective 'romantic' connotes that the partner is chosen based on passionate mutual affection. While cross-cultural studies have suggested that passionate or romantic love is likely to be a human universal (Fisher et al. 2006;Jankowiak and Fischer 1992), it appears that romantic love did not become a normatively expected basis for marriage until the nineteenth century, particularly in Western societies (Coontz 2005;De Munck et al. 2016). This review, like most work in sociology and family demography, focuses on long-term partnerships that may lead to household formation and, potentially, to joint childrearing, rather than on short-term sexual relationships without any commitment. Partner choice encompasses situations in which partners are still living apart, living in unmarried cohabitation, or living together as married spouses. While the main focus of this chapter is on heterosexual relationships, same-sex partner choice is an emerging issue that is discussed in the final part of this chapter (see also Evertsson et al. in this volume). Moreover, the analysis focuses on partnerships that are expected to be monogamous at a given point in time. Polygamy (the practice of having multiple spouses at the same time), either in the form of polygyny (a man having more than one wife) or polyandry (a woman having more than one husband), is not discussed.There is considerable evidence that assortative mating is pervasive (Lichter and Qian 2019; Schwartz 2013). This concept denotes the fact that matches do not occur at random, as partners are sorted based on sharing a number of traits. The concept of endogamy refers to the tendency of people to select a partner within their own social group, while exogamy refers to choosing a partner outside of one's own group (also called intermarriage; e.g., between religious groups). The distinction between endogamous and exogamous unions is used to indicate differences or similarities in qualitative group characteristics such as geographic origin, religious denomination, or ethnic background. Alternatively, the concept of homogamy refers to the tendency of people to choose a partner who has status characteristics close to their own that can be ordered or quantified, such as income, educational attainment, or socio-economic status. The unions of partners who are far apart in terms of such characteristics are called heterogamous. When an individual has a partner with higher status ('partnering up'), the concept of hypergamy is...