Common sense suggests that romantic partners tend to be interdependent in their well-being and self-esteem. The authors tested the degree to which codevelopment in romantic partners (i.e., development in similar directions) is due to mutual influence between partners or due to the effects of shared environment, using longitudinal data from 5 samples of couples (N = 4,116 participants). The samples included dating, cohabiting, and married couples, with a broad age range covering young adulthood to old age. The authors used a longitudinal version of the actor-partner interdependence model and examined measures of well-being (life satisfaction, positive affect, negative affect, and depression) and self-esteem. After conducting the study-level analyses, we meta-analytically aggregated the findings across studies. The results showed significant mutual influence between partners in life satisfaction, positive affect, negative affect, and depression, and significant effects of shared environment on life satisfaction, positive affect, negative affect, depression, and self-esteem. The findings suggest that both mutual influence and shared environment account for codevelopment of well-being in romantic partners. In contrast, only shared environment but not mutual influence contributes to the partners’ codevelopment of self-esteem. The findings have important implications because they show that the level of well-being of a person’s relationship partner has long-term influence on the person’s own well-being.