Provides the first empirical evidence that the U.S. Dept. of Housing and Urban Development's (HUD) tenant-based certificate and voucher programs are already much less likely than public housing to concentrate needy households in poor urban neighborhoods. Evaluates the implementation and effects of existing demonstration and judicially mandated programs that help minority families receiving section 8 certificates and vouchers to move out of areas with high concentrations of minority persons. Charts and tables.