Thesis: This thesis contributes to the analysis of public debt sustainability and fiscal rules. It starts from the multiple empirical evidence that points to the existence of unsustainable fiscal regimes during which fiscal policy does not increase its primary surplus following an increase of public debt. Do unsustainable fiscal regimes necessarily threaten the long-run sustainability of public debt? If not, how long can fiscal policy be periodically unsustainable without being globally unsustainable? The first chapter answers theoretically this question and proposes a Regime-Switching Model­Based Sustainability (RS-MBS) test. We study a Markov-switching fiscal policy rule, which displays an unsustainable fiscal regime, and derive sufficient conditions for the No-Ponzi Game condition and for a globally stable public debt-to-GDP ratio. The second chapter proposes an empirical application of the RS-MBS to France's fiscal policy between 1962 and 2013. It shows that taking into account regime switches can overturn former results and conclude that France's public debt has been sustainable overall the period. The third chapter considers another case of unsustainable regime, when fiscal policy is constrained by the fiscal limit, and studies the effect of public debt maturity on the debt limit. We show that longer debt maturities do not increase the stochastic default threshold when sovereign default is triggered by bad productivity shocks. Finally, the fourth chapter proposes a critical appraisal of the fiscal architecture of the EMU, based on a literature survey about fiscal sustainability, monetary-fiscal interactions and fiscal rules in monetary unions.