The traditional Phillips curve, which shows the relationship between inflation and unemployment, is not stable over time. Tests on data from the US and UK suggest weak evidence of stability in this trade-off. While some stability tests support the Phillips curve, it fails to predict inflation patterns during the Global Financial Crisis. The relationship between inflation and output is not symmetrical, and a non-linear L-shaped model performs best. This shows that relying solely on inflation and unemployment data is not enough to make effective monetary policy, especially during major economic shocks.