The diversity of the applications supported by wireless ad hoc and sensor networks explain the success of these networks. However, their energy constrained nature requires the use of energy efficient strategies to maximize network lifetime. We can classify these strategies in four categories: 1. Energy efficient routing, 2. Scheduling the nodes sleeping state, 3. Topology control by tuning node transmission power and 4. Reducing the volume of information transferred. For each category, a state of the art is given and new protocols are presented. The best energy efficiency will be obtained by combining all the four categories of strategies. A cross-layering approach is a promising trade off between generic solutions and solutions optimized for a specific application.