Battery powered wireless sensor networks (WSN) although can be installed at reduced cost but due to their high powerconsumption and corresponding need for regular battery replacement has made wireless sensor network difficult andcostly to maintain. Energy harvesting is the process by which energy readily available from the environment, and iscaptured and converted into usable electrical energy. Combining WSN nodes with energy harvesters one can approach to even a'Greener' world. This evaluation will make the revolution towards long-lived large scale sensor networks. Thechallenge is in the WSN node integration with an energy harvester, power management, energystorage, communications, and range of sensor types. This paper briefly discusses the trends in energy harvestingtechnologies used for WSN. It also reviews various energy harvesting technologies currently available or underdevelopment. © 2015 IEEE.