Most theoretical models on social dynamics governing development and health across adulthood and old age rely on a "single-unit" approach. We put forth a developmental-contextual model (CoSynch) that utilizes the concept of synchrony (interdependent fluctuations in physiological states and health behaviors) as a novel way to conceptualize social developmental dynamics. Using the sample case of romantic partners, this model aims for an innovative integration of available findings addressing everyday microsocial dynamics in dyads that contribute to overall partner concordance in health trajectories. Specifically, the model builds on 5 propositions to integrate existing empirical findings and infuse future research. Proposition P1: Synchrony is shaped by closeness and shared contexts; P2: Synchrony in physiology and health behaviors follows a u-shaped curve with younger and older adult couples showing greater synchrony than middle-aged couples, but with greater diversification of synchrony in very old age; P3: Couple synchrony is profoundly shaped by life events; P4: Synchrony is correlated with pertinent individual and dyad characteristics; P5: Effects of everyday couple synchrony accumulate over time, thereby carrying significant implications for the health of all involved parties. We also point to open questions and suggest future directions for applying the model that are going to advance knowledge on the social and health dynamics underlying adult development and aging. (PsycInfo Database Record (c) 2021 APA, all rights reserved).