The main scope of this chapter is to present the “Action3” Decision Support System that integrates the MPC+ approach (Map - Plan - Choose - Check), supporting local and regional authorities to the development, implementation and monitoring of the Sustainable Energy and Climate Action Plan (especially within the framework of the Covenant of Mayors). The proposed intelligent system integrates three main modules, namely the “BEI-Action” for the development of the baseline emission inventory, the “MDS-Action” for the creation and evaluation of the alternative Scenarios of Actions aiming at the identification of the most promising Scenario, as well as the “SEC-Action” for the application of the “Methodological Approach for Monitoring SEC (Sustainable Energy Communities) Targets”. The system was developed using the “Java” programming language and the “NetBeans IDE” software development platform. Particular emphasis was laid on the system's design, so as to be user-friendly, combining intuitive menus and navigation throughout the steps of the system.