Although the average rate of unemployment across U.S. metropolitan areas declined between 1980 and 2000, the geographic concentration of the unemployed rose sharply over this period. That is, residential neighborhoods throughout the nation's metropolitan areas became increasingly divided into high- and low-unemployment areas. This paper documents this trend using data on more than 165,000 U.S. Census block groups (neighborhoods) in 361 metropolitan areas over the years 1980, 1990, and 2000; it also examines three potential explanations: (i) urban decentralization, (ii) industrial shifts and declining unionization, and (iii) increasing segregation by income and education. The results offer little support for either of the first two explanations. Rising residential concentration of the unemployed shows little association with changes in population density, industrial composition, or union activity. It does, however, show a significant association with both the degree of segregation according to income as well as education, suggesting that decreases in the extent to which individuals with different levels of income and education live in the same neighborhood may help account for this trend.