We examined the course of maternal depressive symptoms and children's attachment security at 36 months in a large sample of mother-child pairs from 10 sites across the country participating in the NICHD Study of Early Child Care (N = 1077). Maternal depressive symptoms predicted higher rates of insecure attachment. Women who reported intermittent symptoms across the first 36 months had preschoolers who were more likely to be classified as insecure C or D; women with chronic symptoms were more likely to have preschoolers who were classified as insecure D. Symptoms reported only during the first 15 months were not associated with elevated rates of later insecurity. After controlling for potentially confounding demographic variables, maternal sensitivity (observed at 6, 15, 24, and 36 months) did not meaningfully account for links between attachment security and patterns of depressive symptoms. However, the course and timing of maternal depressive symptoms interacted with maternal sensitivity to predict insecurity. Women with late, intermittent, or chronic symptoms who were also low in sensitivity were more likely to have preschoolers who were insecure, in contrast to symptomatic women who were high in sensitivity. These data have implications for understanding the combined impact of maternal depressive symptoms and maternal sensitivity on children's socioemotional development.