This study examined concordance in heart rate (HR) and respiratory sinus arrhythmia (RSA) in a sample of 104 child-maltreating (CM) and nonCM mother–preschooler dyads (208 individuals). In a laboratory setting, mother and child cardiac physiology was simultaneously monitored via ECG in a 5-min resting period. Mothers ranged in age from 20 to 49 years; children ranged in age from 3 to 5 years. Significant within-dyad (WD) and between-dyad (BD) associations were observed for mother HR and both child HR and RSA, and the associations were moderated by CM status. Only CM dyads exhibited BD associations: Higher average maternal HR was associated with higher child HR and lower child RSA. By contrast, when the time interval was divided into 30 s epochs, nonCM dyads exhibited positive WD (dynamic) associations in mother and child HR, and both CM and nonCM dyads showed negative WD associations in mother HR and child RSA. Further, mothers' mean HR levels moderated the extent of epoch-by-epoch WD concordance observed in mother and child, such that elevated average maternal HR was associated with lower levels of WD (dynamic) concordance. No BD or WD concordance in maternal and child RSA was observed. The findings suggest that measures of intraindividual variation provide useful, alternate perspectives in the study of dyadic processes in at-risk families. © 2013 Wiley Periodicals, Inc. Dev Psychobiol 56: 142–152, 2014.