Synchrony, a construct used across multiple fields to denote the temporal relationship between events, is applied to the study of parent-infant interactions and suggested as a model for intersubjectivity. Three types of timed relationships between the parent and child's affective behavior are assessed: concurrent, sequential, and organized in an ongoing patterned format, and the development of each is charted across the first year. Viewed as a formative experience for the maturation of the social brain, synchrony impacts the development of self-regulation, symbol use, and empathy across childhood and adolescence. Different patterns of synchrony with mother, father, and the family and across cultures describe relationship-specific modes of coordination. The capacity to engage in temporally-matched interactions is based on physiological mechanisms, in particular oscillator systems, such as the biological clock and cardiac pacemaker, and attachment-related hormones, such as oxytocin. Specific patterns of synchrony are described in a range of child-, parent- and context-related risk conditions, pointing to its ecological relevance and usefulness for the study of developmental psychopathology. A perspective that underscores the organization of discrete relational behaviors into emergent patterns and considers time a central parameter of emotion and communication systems may be useful to the study of interpersonal intimacy and its potential for personal transformation across the lifespan.