Sur l'efficacité des politiques publiques dans une profonde récession. / In this thesis I evaluate the efficiency of public policies in deep recessions. In the first chapter, I look at the impact of government spending in expansions andrecessionswithamodelthatfeaturesafrictionallabormarket. I show that suchfrictionsarenecessarytoreproducetheasymmetriceffectsofgovernment spending over the cycle observed in the data. The model predicts that higher government spending is useful in boosting output in a liquidity trap not because it generates a lot of inflation, but because it is easier to recruit people in a deep recession. The second chapter looks at the policies implemented by H.Hoover in the beginning of the Great Depression (1929-1941). By dampening the contractionary effects of deflation, the policies of high wages carried out by Hoover mitigated the impact of a large negative aggregate demand shock and reduced the likelihood of undergoing a liquidity trap episode. The third chapter studies the impact of government spending policies on private productioninandoutofaliquiditytrap. Incontrastwiththeliterature,partof governmentspendingisassumedtoenterastockofpubliccapitalwhichitself appears in the production function of the private sector. The multiplier effects of government spending are shown to be higher in normal times. In a liquidity trap, the multiplier effects of investing in public capital are an increasing function of the time to build delay and can be significantly higher than the ones coming from standard wasteful government spending. The fourth and last chapter extends this framework to an open economy. It is shown that the presenceofproductivegovernmentspendingcanreproducetheeffectsofgovernment spending on the real exchange rate.