From a bio-behavioral framework, the relations between physiological synchrony, positive behavioral synchrony, and child self-regulation under varying levels of risk were examined among 93 mother- (M age = 30.44 years, SD = 5.98 years) preschooler (M age = 3.47 years, SD =.52 years, 58.70% male) dyads. Physiological synchrony was examined using interbeat interval (IBI) data and measures of positive behavioral synchrony and self-regulation were based on observations of a mother-child interaction task. Results supported the phenomenon of physiological synchrony among mother-preschooler dyads during an interaction, but not a baseline, task. Moderation analyses indicated that under conditions of high family risk, positive behavioral synchrony and child self-regulation were greater when physiological synchrony was low. Positive behavioral synchrony was positively associated with child self-regulation, regardless of risk status. The results document physiological synchrony among mothers and their preschool-aged children and the complex ways that physiological attunement relates to important developmental processes.