A well-functioning tax system requires that everyone pays the taxes they owe. Today, the “tax gap”—the difference between taxes that are owed and collected—totals around $600 billion annually and will mean approximately $7 trillion of lost tax revenue over the next decade. The sheer magnitude of lost revenue is striking: it is equal to 3 percent of GDP, or all the income taxes paid by the lowest earning 90 percent of taxpayers.