In recent years, a new wave of network labeled Wireless Sensor Networks (WSNs) has attracted a lot of attention from researchers in both academic and industrial communities. WSNs consist of tiny, energy efficient sensor nodes communicating via wireless channels, performing distributed sensing and collaborative tasks for a variety of monitoring applications. They include physical phenomena like temperature, humidity, vibrations, seismic events and pollution detection, to feature extractions. An energy source supplies the energy needed by the device to achieve the programmed assignment. This energy source often consists of a battery with a limited energy plan. It could be unfeasible or difficult to recharge the battery, because nodes may be deployed in a hostile or unpractical environment. The sensor network should have a life span long enough to fulfill the application requirements. Therefore, the crucial issue is to prolong the network lifetime. In some cases, it is possible to scavenge energy from the external environment (e. g. by using solar cells as the energy source). However, external energy supply sources regularly exhibit a non-continuous performance so that an energy buffer (a battery) is needed, as well. In any case, energy is a very significant resource and must be used very sparingly. Therefore, the main issues in wireless sensor networks is how to prolong the network lifetime of WSNs with a certain energy source and how to maintain coverage and connectivity. Optimizing the energy consumption in wireless sensor networks has recently become an important concern. Hence, energy management is a key issue in Wireless Sensor Networks.