The nearly nine million duals who receive Medicare and Medicaid benefits constitute one of the nations most vulnerable and costly populations. Several initiatives authorized by the ACA are intended to improve the care delivered to duals and achieve greater control of spending growth for Medicare and Medicaid. We examined the 2007 costs and service use associated with duals. Although the population is indeed costly, we found nearly 40 percent of duals had lower average per capita spending than non-dual Medicare beneficiaries. We also found that about 20 percent of duals accounted for more than 60 percent of combined Medicaid and Medicare spending on duals. But even among high-cost duals, we found subgroups. Findings suggest that decisionmakers should tailor reforms to account for subpopulations of duals.