The transition of our current energy system from a fossil-based system to a system based on renewables is likely to be one of the most complex and long-term societal transitions in history. The need for a fundamental system transformation raises the question of how to measure the continuing progress and the resilience of this process over time. We present a conceptualization and operationalization of resilience for energy systems in transition with regard to both social and technical aspects. Based on the resilience concept in social-ecological systems literature, we propose to conceptualize resilience for energy systems building on two core attributes of resilience, namely diversity and connectivity. We present an indicator set to operationalize these key attributes in social and technical systems using definitions and measurements for three fundamental diversity properties—variety, balance and disparity—and basic connectivity properties from the social network analysis literature—path length, centrality and modularity. Furthermore, we show first empirical applications on case studies on energy regions in Bavaria, Germany and Styria, Austria – based on a mixed-methods approach. Finally, we reflect on challenges and the added value of the approach for research on resilience of energy systems and policy making.