This tutorial presents the foundations, techniques and tools on energy debt. Energy debt adapts the technical debt metaphor to the green software realm. Thus, we define energy debt as the additional estimated energy cost of executing a software system, due to energy consumption inefficiencies in the software’s source code, when compared to the estimated energy cost of executing the energy-optimal version of that same software. To express energy debt we compiled a catalog of Android-specific energy code smells which are presented in the current state-of-the-art literature on green software, together with the energy savings reported in the studies where such smells have been presented. Finally, we present the E-Debitum tool, an energy debt estimation plugin for SonarQube that can be used by developers and researchers to estimate energy debt on Android applications.