Abstract : The problem of restoring power Systems after a complète or partial blackout is as old as the power industry itself. In récent years, due to économie compétition and deregulation, power Systems are operated closer and closer to their limits. At the same time, power Systems hâve increased in size and complexity. Both factors increase the risk of major power outages. After a blackout, power needs to be restored as quickly and reliably as possible, and consequently, detailed restoration plans are necessary. This thesis concentrâtes on the initial stages of restoration where four major problem areas are identified. A method to détermine optimal units start-up séquence during power plants re-start up is presented. The system frequency behavior after the energization of loads is assessed using a novel approach. An approach to standing phase angle réduction is explained. The occurrence of overvoltages is assessed in the frequency domain. Sensitivity analysis is used in order to find the most efficient network change that can be applied to limit overvoltages. In case time-domain simulations need to be carried out, a method based on Short-Time Fourier Transform helps to limit the overall calculation time. Ail the proposed approaches are validated using time-domain simulations based on IEEE test Systems data. Also a blockset in MATLAB/SIMULINK environment has been developed for long term dynamics simulation to study frequency and voltage responses due to load and génération mismatches for analysis power system restoration scénarios.