As of December 2008, state governments had approximately $1.94 trillion set aside in pension funds for their employees. How does the value of these assets compare to the present value of states' pension liabilities? Just as future Social Security and Medicare liabilities do not appear in the headline numbers of the U.S. federal debt, the financial liability from underfunded public pensions does not appear in the headline numbers of state debt. If pensions are underfunded, then the gap between pension assets and liabilities is off-balance-sheet government debt. We show that government accounting standards require states to use procedures that severely understate their liabilities. We then discuss the true economic funding of state public pension plans. Using market-based discount rates that reflect the risk profile of the pension liabilities, we calculate that the present value of the already-promised pension liabilities of the 50 U.S. states amount to $5.17 trillion, assuming that states cannot default on pension benefits that workers have already earned. Net of the $1.94 trillion in assets, these pensions are underfunded by $3.23 trillion. This “pension debt” dwarfs the states' publicly traded debt of $0.94 trillion. And we show that even before the market collapse of 2008, the system was economically severely underfunded, though public actuarial reports presented the plans' funding status in a more favorable light.