Establishing and repairing ruptures in mother-infant positive affect synchrony are thought to support infants' self-regulation and social competence. Yet, despite rich theorizing, little is known about associations between mother-infant positive affect synchrony and emotion coregulation. This study used the Still Face Paradigm (SFP), which consists of separate initial play, Still Face (SF) stressor, and reunion phases, to assess levels and changes of within-dyad synchrony before and after an experimentally induced relational stressor in a sample of 77 mother-infant (