This paper gives an overview of energy harvesting techniques for generating electrical power from ambient energy such as vibration, heat and light. Typically, the electrical power is used to energise sensing systems and their associated communication channels, which are often wireless in nature. The amount of electrical power generated is in the region of tens of micro watts to hundreds of milli watts. The use of assets in service generally results in some form of ageing as a consequence of elevated temperature, chemical degradation, absorption of energetic radiation, mechanical wear, etc. and, as a consequence, as time progresses, the probability of failure increases. In some situations, it may be acceptable on grounds of cost and consequent impact to accept periodic failure and asset replacement while, in many others, such a strategy is unacceptable such that regular maintenance, condition assessment, life prediction and pre-emptive replacement must be carried out. A clear example of this is in connection with mass transport, notably aviation, where the impact of catastrophic failure can be dire. Condition assessment may be achieved through periodic examination or by continuous monitoring but, in many circumstances, the latter approach has much to recommend it. This commonly involves the measurement of relevant parameters and the subsequent export of data for decision making. Such systems require sensors, data logging, communications hardware, etc. plus a source of energy. A broad overview of energy scavenging techniques is presented, along with an assessment of useful materials for each mode of harvesting.