This review of liquidity traps unifies three landmark economic downturns—the US Great Depression, the Great Recession, and Japan’s Long Recession—into a single analytical framework. We examine various forces that drive natural interest rates negative: temporarily (such as banking crises and debt overhangs) or permanently (such as demographic shifts and inequality). When policy rates hit the zero lower bound, conventional monetary tools lose traction. Under a standard monetary policy regime, counterintuitive paradoxes emerge: Greater price flexibility deepens recessions, and positive supply shocks become contractionary. We show how policy effects—including the size of fiscal multipliers, forward guidance, and these paradoxes—depend critically on the monetary-fiscal regime and on central bank credibility. The paper explains how regime changes, such as Franklin D. Roosevelt’s 1933 abandonment of the gold standard and balanced-budget dogmas, successfully reversed deep slumps by credibly shifting expectations. We examine whether secular-stagnation forces are likely to assert themselves in the coming decades.