Parent-infant synchrony is as a dynamic, bi-directional, multimodal process through which partners mutually adjust their physiological, neural, and behavioral states in time. Here, we systematically reviewed the available evidence on parent-infant physiological and neural synchrony during the first 12 months of life. Out of 3786 records initially identified, 42 studies met our inclusion criteria: 27 employed peripheral physiological measures (e.g., ECG, respiration) and 15 utilized central neural measures (e.g., EEG, fNIRS). Studies relying exclusively on behavioral indices, hormonal samples, or single-participant intrapersonal coupling were excluded. Findings reveal that parent-infant synchrony emerges as a dynamic, context-dependent process in which autonomic and neural signal flexibly adapt to the emotional demands of the interaction, becoming most robust during embodied, multisensory, and affectively charged episodes. Further, synchrony functions neither as uniformly protective nor risky but as a bi-directional, multimodal process that can support resilience or reflect co-dysregulation. Its magnitude depends on parental characteristics, dyadic relationship quality, and interactional context. Current parent-infant synchrony research is limited by methodological constraints, small and homogeneous samples, heterogeneous preprocessing pipelines, and difficulties aligning physiology, behavior, and neural data, highlighting the need for multi-level, longitudinal, and culturally diverse designs. Overall, this review underscores synchrony as a dyadic, bidirectional process that emerges across the first year of life and lays the foundation for socio-emotional development. It warrants further investigation through expanded research designs, greater population diversity, and multimethod approaches to better elucidate its cascading effects on the parent-child relationship and child well-being.