A framework for the modeling and simulation of transitions is presented. A transition, “substantial change in the state of a socio-technical system”, typically unfolds over a long timespan. We therefore suggest to use simulation to inform transition managers on the effect of their decisions. Transition models preferably meet five functional requirements: to allow for the representation of physical and social components, for material and immaterial interactions, to allow the system structure to change, to compute transition indicators and to capture the effect of interventions. Modeling the decarbonization of the power sector illustrates that an agent-based model allows us to let both the content and the structure of a system to emerge in a simulation.