In September 2007, child support arrears had reached $107 billion. The purpose of this report is to provide information about the individuals who owe child support arrears, estimate how much arrears are likely to be collected, and identify the factors that have led to their rapid growth. We find that most arrears are owed by noncustodial parents who have no or low reported incomes. We estimate that 40 percent of arrears in seven large states are likely to be collected over 10 years. The primary reason arrears have grown is because many states have begun to assess interest on arrears.