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Harvesting small scale energy from otherwise wasted ambient energy sources has attracted immense research efforts for battery-powered wireless sensor networks for various applications such as structural health monitoring, industrial condition monitoring and healthcare. The power level which those applications may reach ranges from microwatts to milliwatts. Energy scavenged from ambient sources may be able to recharge or even eliminate the battery to power up those devices perpetually. Sources of energy for harvesting include, but are not limited, to light, thermal gradient, vibration, and radio frequency radiation. The fluctuation and intermittence of these ambient energy sources bring in challenging technical issues to make self-powered systems practical. Energy storage devices like rechargeable batteries or supercapacitors and efficient power management circuitry are indispensable to convert a dynamic environmental energy input into a stable power source. This presentation reviews principles of energy harvesting and practices for small scale energy harvesters and self-powered wireless sensor modules developed recently. Industry trends and possible research issues for further developments are discussed in order to give a technical insight into energy harvesting techniques and their applications.