It is well established that parent-child dyadic synchrony (e.g., mutual emotions, behaviors) can support development across cognitive and socioemotional domains. The advent of simultaneous two-brain "hyperscanning" (i.e., measuring the brain activity of two individuals at the same time) allows further insight into dyadic "neural synchrony." In this article, we review 16 recent studies of naturalistic, parent-child brain-to-brain synchrony, finding relations with the nature of interactions (collaborative vs. competitive, parent vs. stranger), proximal social cues (gaze, affect, touch, and reciprocity), child-level variables (irritability, self-regulation), and environmental factors (parental stress, family cohesion, and adversity). We then discuss how neural synchrony may provide a biological mechanism for refining broader theories on the developmental benefits of dyadic synchrony. We also highlight critical areas for future study, including examining synchrony trajectories longitudinally, including more diverse participants and interaction contexts, and studying caregivers beyond mothers (e.g., other family members, teachers). We conclude that neural synchrony is an exciting and important window into understanding how caregiver-child dyadic synchrony supports children's social and cognitive development.