In our conceptual paper, we develop a systemic social mechanisms model to explain change and inertia of energy systems. Situational, action-formation, and transformational mechanisms that drive change in a transition require corresponding framing and framing contests to create legitimacy for that transition. We conceptualize mechanisms of socio-technical transitions and of creating legitimacy for transitions as mutual drivers and outcomes, with framing contests as crucial for achieving legitimacy for change. We propose that the social mechanisms approach supports evidence-based policy-making, underlines the need for flexibility in the face of changing contexts, and highlights the key role of framing contests for meaning making and for activating further mechanisms. We illustrate our proposition with two examples, the Polish and the German electricity system.