In wireless ad hoc and sensor networks, some nodes are battery operated. They have a limited
amount of energy that can be difficult, expensive or even impossible to renew like in forest fire detection,
nuclear plant monitoring or explorations in hostile environments. The challenge for these networks consists
in maximizing network lifetime by means of energy efficient techniques. In this chapter, we first introduce
basic concepts with regard to energy efficiency and present the four classes of energy efficient techniques:
energy efficient routing, node activity scheduling, optimizing the transferred information and adapting
transmission power to the topology. We then focus more particularly on energy efficient routing and
classify existing protocols according different criteria: data centric, hierarchical, geographical, energy
criteria used for route selection, multipath routing and support of sleeping nodes. We present EOLSR, the
energy efficient extension of the OLSR routing protocol supporting different types of networks like IEEE
802.11 and IEEE 802.15.4. We compare its performance with multipath routing. Node energy consumption
can be considerably reduced by scheduling node activity. We will briefly present SERENA that increases
not only the network lifetime but also the amount of user data delivered. Hence, SERENA contributes to a
more efficient use of energy and can be used with any routing protocol. To design a network protocol, two
approaches have been traditionally used: a generic one that can lead to poor performance for some
applications and the opposite one, specifically designed for a given application (e.g. data gathering). An inbetween
approach taking into account application specificities and environmental constraints by means of
cross layering provides a very promising trade-off leading to improved performance and reactivity. We
show how EOLSR benefits from cross layering with (1) the MAC layer by a better reactivity to topology
changes and (2) the application layer by a reduced overhead. This induces an increased network lifetime
