The article looks at Japan's experience in the 1990s to understand how to prevent deflation. It explores when an economy might enter a period of sustained deflation, how quickly monetary policy should react to falling inflation, and the role of fiscal policy in fighting deflation. The study shows that Japan's policymakers didn't expect the long-lasting deflation, which made it hard for them to boost growth and inflation. When inflation dropped and interest rates neared zero, it became tough for monetary policy to kickstart the economy. The research suggests that when inflation and interest rates are close to zero, and the risk of deflation is high, both monetary and fiscal stimulus should be stronger than usual forecasts predict.