It is well known that Medicaid spending per beneficiary varies widely across states. However, less is known about the cause of this variation, or about whether increased spending is associated with better outcomes. In this article we describe and analyze sources of interstate variation in Medicaid spending over several years. We find substantial variations both in the volume of services and in prices. Overall, per capita spending in the ten highest-spending states was $1,650 above the average national per capita spending, of which $1,186, or 72 percent, was due to the volume of services delivered. Spending in the ten lowest-spending states was $1,161 below the national average, of which $672, or 58 percent, was due to volume. In the mid-Atlantic region, increased price and volume resulted in the most expensive care among regions, whereas reduced price and volume in the South Central region resulted in the least expensive care among regions. Understanding these variations in greater detail should help improve the quality and efficiency of care-a task that will become more important as Medicaid is greatly expanded under the Affordable Care Act of 2010.