--- - |2 Abstract In this article, I examine when neighborhoods matter for children's development. I review conceptual and empirical work on developmental timing, highlighting the dynamic interactions between neighborhood context and children's development in the first two decades of life, and incorporating the issue of residential mobility (children moving across or within neighborhoods). This research suggests that childhood may be a time when individuals’ achievement is sensitive to neighborhoods’ socioeconomic conditions, while moving in middle childhood or early adolescence may be most consequential for children's development, both in terms of achievement and socioemotional functioning. In both cases, cumulative exposure to neighborhood socioeconomic conditions or residential mobility in the first two decades of life may be more important for children's development than exposure during either childhood or adolescence, but it likely depends on their co‐occurrence and the outcome. I conclude by addressing methodological considerations for further study. - Child Development Perspectives, EarlyView.