This meta‐analytic review (k = 5–10; N = 258–895) examined links between attachment insecurity and physiological activity at baseline and in response to interpersonal stress elicited by separation–reunion procedures in the early life course (1–5 years). Insecurity was trivially, nonsignificantly associated with baseline physiological activity (heart rate [HR]: g = −.06; respiratory sinus arrhythmia [RSA]: g = −.06; cortisol: g = .01) and nonsignificantly associated with physiological reactivity to separation from parents (HR: g = −.001; RSA: g = .24). However, insecurity was moderately associated with heightened RSA (g = .26) and cortisol (g = .27) reactivity upon reunion with parents. Findings provide insight into the biobehavioral organization of attachment, suggesting that early insecurity is associated with heightened physiological reactivity to interpersonal stress.