Capitalism and interest are inseparable, yet over the centuries whenever interest rates have collapsed and money was too easy, financial markets have become unstable. In the first two decades of the twenty-first century, interest rates have sunk lower than at any time in the five millennia since they were first recorded. In an unprecedented move, negative interest rates were introduced in Europe and Japan, causing trillions of dollars' worth of bonds to trade at negative yields. Monetary policymakers appear blithe to the unintended consequences of their actions. Yet given the essential function of interest in determining how capital is allocated and priced, and its role in regulating financial risk, it is not clear that capitalism can thrive or even survive under these conditions.