Dissimilarities between partners in prominent domains of functioning are often thought to be a risk factor for compromised relationship quality and relationship dissolution. However, the nature, correlates, and consequences of developmental trajectories of within-couple dissimilarities in key quality-of-life indicators such as life satisfaction are not well understood. In the current study, we applied multilevel growth models to up to 31-wave annual longitudinal data from 13,714 romantic partners in the German Socio-Economic Panel (age at baseline: M = 43 years, SD = 15, range 17–92 years). Partner dissimilarity was calculated at the within-couple level and indicated considerable differences in life satisfaction between partners within a given couple (0.64 SD or 1.14 units on an 11-point scale). Over time, partner dissimilarity slightly increased among partners who remained together. Examining individual and relationship correlates indicated that dissimilarity was greatest for couples who were older, had children, or had a shorter relationship history. Also, dissimilarity was greater when individual life satisfaction or satisfaction with family life was low, particularly among wives, as well as among couples who later separated. Examining consequences, larger levels of and increases in partner dissimilarity were independently predictive of lower satisfaction with family life at the end of the study, over and above individual life satisfaction of either partner as well as key individual and relationship correlates. Our discussion focuses on the advantages of investigating (developmental trajectories of) within-couple dissimilarity and its implications for individual and partner development.