Measuring the energy consumption of a communication device is necessary in order to develop energy aware and energy efficient protocols and applications. There exist various approaches to determine the energy consumption of a device. This paper gives a structured overview on the state of the art of energy measurement and provides a detailed analysis of the advantages and drawbacks of each technique. An improved understanding of how energy measurement can actually be accomplished allows to choose the right measurement technique according to a particular purpose and leverages the development and improvement of energy aware and energy efficient protocols and applications.