Data from the National Longitudinal Survey of Labor Market Experience for Youth (1997 cohort) are used to examine the urban school achievement gap. Specifically, we use the Blinder-Oaxaca technique to decompose differences in Armed Services Vocational Aptitude Battery scores for students who attended urban and suburban schools. We find that approximately 75% of the gap in this achievement measure is explained by the high concentration of disadvantaged students in urban schools. Broken down further, 36% of the gap can be attributed to differences in family background. The lower income of urban families alone explains 25% of the gap. Differences in measures of school quality, such as small classes, large schools, and private school attendance, explain very little of the gap. While current policy focuses on schools and school reform, our results are a reminder that meaningful efforts to improve performance in urban schools must address socioeconomic conditions in urban areas. (Contains 5 tables and 3 notes.)