In this thesis, we study the development of distributed cyber-attack detection methodologies with plug-and-play characteristics, to monitor large-scale cyber-physical systems, with a particular focus on future energy distribution networks. Over the last few decades, the penetration of distributed generation capacity in power networks has greatly increased, especially that of renewable energy sources. Thus, to maintain the required performance, novel methods for voltage and power regulation have been suggested. Given their scale and complexity, distributed control architectures have been proposed to regulate these networks, where the system is divided into subsystems, each with its own controller. These may then communicate to achieve global objectives. This has been in large part made possible by the advances in ICT and power electronics. Furthermore, the integration of cyber-resources in other systems has been a trend also in other critical systems. The increasing number of cyber-physical systems, however, implies the possibility of malicious agents interfering with the operations of critical infrastructure, thus causing significant disruptions, as attested by some high profile cases in recent years. Moving from this motivation, it is the primary objective of this thesis to define distributed cyber-attack detection architectures capable of detecting the presence of malicious agents: drawing upon a distributed control architecture for DC microgrids as a motivating example, we suppose that attacks may penetrate the communication network included between subsystem controllers to achieve current sharing. Moreover, we endow these distributed diagnosers with plug-and-play capabilities, i.e. the ability of automatically being reconfigured if the system changes its configuration. These techniques are validated through simulation on DC microgrids, an energy distributed network where generation and consumption of electricity are colocated within a single node, and experimentally on a network of electric springs, a demand-side management technology capable of regulating the voltage and power quality over distribution networks.