Children commonly experience pain, fear, and distress from needle procedures, which if left unmanaged, can contribute to short and long-term consequences, including increased risk of injury, greater pain at future procedures, increased fear, and healthcare avoidance. For parents, pediatric needle procedures can evoke responses oriented to the self (e.g., avoidance) or their child (e.g., approach behaviour). Biopsychosocial frameworks are widely applied to understand children’s and parents’ experiences with pediatric needle procedures; however, internal physiological regulation, which can be acquired without disrupting natural interpersonal dynamics, is largely absent from empirical investigation. The goal of this dissertation was to examine biopsychosocial contributors to parent and child (7-12 years) experiences with pediatric venipuncture, with an emphasis on heart rate variability (HRV) as a physiological index of emotion regulation. Stemming from a randomized controlled trial investigating a mindfulness intervention for children and parents before venipuncture, this dissertation presents the results from three studies, which are described in three manuscripts. Study 1 (Constantin, Moline, et al., 2022) examined: 1) parent and child HRV throughout venipuncture and whether a brief, randomly-assigned audio-guided mindfulness vs. control exercise affected this pattern; and 2) the extent to which changes in parent and child HRV were synchronized throughout venipuncture. Practicing the mindfulness vs. control exercise did not consistently affect HRV and a pattern of non-synchrony emerged. Study 2 (Constantin et al., under review) examined parent HRV, state catastrophizing, anxiety, and child behaviours as contributors to parent verbal behaviours throughout venipuncture. Baseline HRV moderated the association between parent catastrophizing and distress-promoting behaviour, such that the association was strongest among parents with low HRV. Study 3 (Constantin, Lupo-Flewelling et al., 2022) examined child HRV, pain, fear, and distress, and a constellation of parent behaviours to understand children’s venipuncture experience. Child HRV moderated the association between a group of parent behaviours (reassurance, empathy, giving control) and child distress; the association was strongest among children with low HRV. Together, this research provides foundational knowledge regarding the patterns of parent and child internal regulation throughout pediatric venipuncture and how internal regulation furthers our understanding of parent and child experiences during a common needle procedure.