This issue of Cityscape focuses on the Moving to Opportunity (MTO) for Fair Housing demonstration, which provided selected low-income households in public housing with vouchers to move from high-poverty to low-poverty areas and compared the outcomes of those households to those of a control group. In the first articles, the design of the experiment and the rationale behind the MTO demonstration are described, specifically outlining the interest in the experiment as a study of neighborhood effects on life outcomes and as a response to residential segregation and the concentration of poverty. The following articles analyze the effects of the experiment on participating adults and youth and discuss the relevance of the demonstration outcomes to current and future housing policy. Raising an issue pertinent to the determination of concentrated poverty and residential segregation, several articles also discuss the difficulty of defining neighborhoods and the appropriateness of often-used proxies for neighborhoods, such as census-defined geographies.