In Switzerland, Germany and Austria, as in many European countries, the transition of the energy sector towards more sustainability is a long-term transition process, unfolding since decades. It however only reached the regime level in recent years and therewith the established technologies and governance structures of the energy systems. This fundamental transition process causes complex, interrelated and non-linear dynamics and changes on multiple scales in the technical and social sphere of the energy systems. The energy transition, thus, became a mature sustainability transition, which is a new challenge for research and decision-making in practice. This dissertation contributes to an improved conceptual and empirical understanding of actor- and system-level structures and dynamics in mature sustainability transitions. To tackle this research aim, the dissertation employed an iterative theory building process, which develops theoretical considerations based on preexisting theories and frameworks as well as empirical evidence. The empirical analyses were thereby conducted using a mixed methods approach, which allowed for rich empirical evidence and triangulation. The research was implemented in four modules, whereby each module tackled one research question. Main data sources were scholarly literature, regional structural data and documents as well as transcripts from several rounds of semi-structured expert interviews and expert workshops. The empirical data stemmed from three cases: energy regions in Austria and Germany, a network of change agents in Germany and urban utility companies in Germany and Switzerland. The main analytical methods, employed in this dissertation were qualitative literature analysis, document analysis and structuring qualitative content analysis. For the analysis of system structure for functionality and transition dynamics, this dissertation presents an indicator set to analyse and measure resilience of sociotechnical energy systems in transition, based on the key system characteristics of diversity and connectivity. The empirical application provided rich insights on the sociotechnical system structure and its changes over time and discusses appropriate methods for the empirical analysis. For the analysis of transition dynamics, this dissertation provides a reconceptualised framework for the systematic analysis of actor- and system-level determinants of agency as well as the feedback of agency on the system. Especially focusing on the role of incumbents in mature sustainability transitions, the dissertation moreover provides findings from the empirical exploration of urban utility companies and derived analytical perspectives for public incumbent actors in network industries in transition. The dissertation finally discusses the integrability of conceptual findings to transition studies and presents an integrated framework to analyse structures and dynamics in mature sustainability transitions. The thesis provided key theoretical and empirical insights on actor- and system-level aspects of mature sustainability transitions in sociotechnical energy systems. It contributed to a further discussion on dynamic resilience concepts for sociotechnical energy systems, a more systematic analysis of agency in sustainability transitions and a more nuanced picture of incumbents' agency in sociotechnical energy systems in mature sustainability transitions.