Researchers over the past two decades have increasingly recognized the importance of neighborhood contexts for youth development. For example, living in a disadvantaged neighborhood has been associated with a wide range of negative outcomes throughout the early years of the life course. However, neighborhoods likely have very different effects, depending on a family's socioeconomic resources, a youth's age, gender, and many other characteristics. Using a life course and ecological perspective as an organizing framework, this article looks at how the characteristics of neighborhoods, families, and children interact to shape development over time. Special attention is paid to the types of social relationships within and between neighborhoods. A conceptual model of neighborhood effects on youth development is provided, and prevention implications are given. (Contains 1 figure.)