The Highway Trust Fund is the principal mechanism for funding federal highway and transit programs through receipts from excise taxes charged to highway users, such as taxes on motor fuels. The Department of Treasury (Treasury) and the Congressional Budget Office (CBO) each prepare estimates of future receipts for the Highway Trust Fund semiannually. Treasury's receipt estimates are combined with the Department of Transportation's (DOT) estimates of outlays to create an estimate of the Highway Trust Fund balance for the President's Budget; CBO also projects outlays to develop an estimate of the fund balance. The agencies' most recent estimates show that the Highway Account within the Highway Trust Fund could have a negative balance as early as 2009, raising concerns about whether funding for federal highway programs--which were recently authorised by the Safe, Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users--will continue to be met. Consequently, the Subcommittee asked us to review and compare recent estimates made by Treasury and CBO. This testimony provides information on how (1) estimates are used to provide key information about the Highway Trust Fund, (2) the most recent Highway Trust Fund estimates -- based on receipt estimates made by Treasury and CBO--compare, and (3) Treasury's and CBO's estimates compare to actual receipts for recent years.