This thesis presents two works on interventions in period of crisis. The first one presents a new policy involving a relaxement of the collateral requirement and a government participation sharing the commitment of the agents in the financial assets. Numerical results show that this policy can lead to a Pareto improvement in some economies in which the unconventional monetary policy does not lead to. In the second work a numerical analysis is developed on the effects of two crisis interventions, the unconventional monetary policy and the new policy presented, in a context of heterogeneous beliefs. The numerical results indicates that the relative optimism is important to determine the constraints of the agents and that the unconventional monetary policy is potentialized with the relative optimism of the poor agent. Furthermore, the results suggests that the policy proposed in the first work is preferable when the poor agent is relatively pessimistic and the unconventional monetary policy is preferable when he is relatively optimistic.