Over the last decade, observers of state and local finances have been alarmed over an emerging picture of long-term, structural imbalances. This article examines the concept of fiscal sustainability in several formulations and explains that it essentially means limiting expenditure commitments to those that can be met by available revenue streams. It investigates why fiscal sustainability in actual practice, however it might be measured in theory, has fallen into disrepair. The usual lineup of budget-busting culprits is next examined, with the proliferation of entitlement programs standing at the head of the line. Over the past four decades, state and local budget increases reflected the strength of the economy during an unprecedented run of prosperity. Meanwhile, the array of entitlement programs that drove spending was increasingly shaped by political, demographic, and institutional forces, each with its own clientele of beneficiaries. That has made adjustments more difficult when revenues do not keep pace with spending patterns.