--- - |2 Abstract There is a paucity of developmental data examining the stability of autonomic function during childhood, and even less is known regarding the extent of coordination between autonomic branches. To address this, autonomic measures of sympathetic (cardiac pre‐ejection period, PEP; electrodermal activity, and EDA) and parasympathetic (respiratory sinus arrhythmia and RSA) function were collected from 339 children annually from kindergarten to second grade. Physiology was quantified into 31 epochs 30 s in length, across a series of baselines and emotion‐inducing films. Rank‐order stability was observed for all three indices, but was higher for cardiac than electrodermal measures. A series of multilevel models was used to identify the proportion of variance in psychophysiological function attributable to the individual (trait), visit (potential developmental change), or epoch (reactivity to task). Both PEP and RSA had sizeable components of individual‐level variance, with PEP showing very small variance attributable to experimental task. In contrast, variance in EDA was largely attributable to task epoch. Variance decompositions of the parasympathetic–sympathetic coordination (e.g., RSA‐EDA) suggest that the branches of the autonomic system are prototypically cooperative, but evidence did not indicate traitlike stability or linear developmental change. The extent of coordination was moderated by emotional context, indicating that autonomic coordination varies dynamically within individuals. These findings have implications for future work attempting to identify the contextual, experiential, and demographic factors that modulate developmental trajectories of autonomic function and coordination, and for examination of how developmental changes in autonomic psychophysiology are related to or predict behavioral trajectories across a range of psychological domains. - Psychophysiology, Volume 55, Issue 11, November 2018.