Large-scale decentralized systems organized in overlay networks are complex to manage. Such systems integrate organizational complexity in the application-level resulting in low abstraction and modularity in their services. This thesis introduces a multi-level conceptual architecture for overlay services. An overlay service is a large-scale decentralized system organized in overlay networks that provides generic application capabilities. Two overlay services introduced in this thesis provide a proof-of-concept for the higher abstraction and modularity that can be achieved with this architecture. The first overlay service builds and maintains overlay networks that are self-organized in different tree topologies according to different application criteria. The second overlay service performs a function-indepencent and routing-independent decentralized aggregation of values that are distributed in a network and continuously changing. Experimental evaluation studies the performance trade-offs imposed by this generic design. This thesis studies overlay services in the application domain of the Smart Power Grid. More specifically, overlay services are used in demand-side energy management to perform (i) load-shifting and (ii) demand-adjustment in a fully decentralized fashion. The evaluation of this approach is grounded in the current reality and practice of the Smart Power Grid.