The end of cheap oil has created a new sense of urgency: we must increase our understanding of the underlying mechanisms of system transitions and learn how to steer complex and large socio-technical systems such as the energy infrastructure. To this end, we present a framework for assessing the system transition process using simulation. This consists of (1) unravelling the interaction and feedback between system levels and actors (2) determining the role of relevant actors (3) defining suitable transition indicators (4) developing a simulation model. Assessing the energy system transition induced by the European carbon policies illustrates the use of this framework. Therein, net CO2 emission and the power generation portfolio are deemed as transition indicators.