High public debt is widely perceived as economically, and even morally, destructive. Leaving aside the nearly religious arguments about debt and sin, two economic reasons are typically given. The first is fiscal costs: High debt implies high distortionary taxes in the future. The second is welfare costs: Debt crowds out capital in the portfolios of savers, decreasing capital accumulation and thus decreasing future output and consumption. In the current environment of low interest rates, authors wondered how large these costs were and decided to explore that in my address.