This dissertation estimates patterns of assortative mating for body weight using two underutilized dyadic datasets: the Marital and Relationship Survey (MARS) and the Add Health Romantic Pairs data. The dissertation includes three distinct chapters. The first chapter examines weight concordance among dating, cohabiting, and married young adult couples and provides an empirical test of the winnowing hypothesis. The winnowing hypothesis asserts that progression of intimate relationships toward marriage is marked by increasing levels of homogamy among partners. This Add Health study utilizes log-linear models to identify associations in cross-classifications of male partner and female partner BMI status data (underweight, healthy weight, overweight, obese) while simultaneously controlling for the marginal distribution of male and female partners' BMI status by union type. Results demonstrate evidence of "reverse winnowing": Dating couples exhibit the strongest BMI status concordance, followed by cohabiting and then married couples. Consistent with a stigma effect, obese individuals are more likely to be in weight concordant unions. The second chapter examines spousal differences in BMI. Mechanisms for BMI concordance and discordance are reviewed and a female-thinner norm is proposed. In MARS data, multilevel and fixed effects regression are used to statistically control for passive matching effects of social homogamy and convergence and to address unobserved heterogeneity between spouses. Results suggest wives tend to significantly outweigh their husbands although not by much. BMI differences among higher educated husbands and wives are consistent with a female-thinner norm. Studies that fail to control for unobserved heterogeneity may report biased spousal associations. The third chapter (coauthored with John Cawley, Kara Joyner, and Jeffery Sobal) uses Add Health data to examine both physical and sociodemographic characteristics young adults trade in order to obtain a physically attractive romantic partner. Focusing on one specific correlate of attractiveness -- body weight -- the study shows obesity reduces the likelihood of being matched with a physically attractive romantic partner, particularly among white women. Women, like men, trade education status for a physically attractive partner. Despite new evidence presented in this dissertation, assortative mating for body weight is a complex social phenomenon that has yet to be fully explored.