The objective of the GRE3N project is to propose cross-layer techniques that integrate the different layers and levels of a communications system (component, physical, link, medium access, and system) and that are energy-efficient. These techniques will take into account those elements in the network consuming most energy and those that are most limited in terms of state of the batteries. Besides, not only the transmission power will be considered, but also all the possible sources of energy consumption, as well as methods for energy harvesting to charge the batteries.