Trajectories of emotion regulation processes were examined in a community sample of 269 children across the ages of 4 to 7 using hierarchical linear modeling. Maternal depressive symptomatology (Symptom Checklist-90) and children's physiological reactivity (respiratory sinus arrhythmia [RSA]) and vagal regulation ([delta]RSA) were explored as predictors of individual differences in trajectories of emotion regulation and negativity (mother-reported Emotion Regulation Checklist; A. M. Shields & D. Cicchetti, 1997). In addition, the authors explored whether children's physiological regulation would moderate the effect of maternal depressive symptomatology on children's emotion regulation trajectories. Results indicated that over time, emotion regulation increased whereas negativity decreased, though considerable individual variability in the pattern of change was observed. Greater maternal depressive symptomatology was associated with less steep emotion regulation trajectories. There was a significant Maternal Depressive Symptomatology [times] Baseline RSA [times] Age interaction predicting emotion regulation trajectories. Overall, it appears that the development of emotion regulation over time is compromised when mothers report greater depressive symptomatology. There is also evidence that children's capacity for physiological regulation can buffer some of the adverse consequences associated with maternal depressive symptomatology. (Contains 5 tables, 2 footnotes, and 3 figures.)