As human dependence on electronic devices grows, there is an emerging need on finding sustainable power sources for low power electronics and sensors. One of the promising possibilities in this space is the human body itself. Harvesting significant power from daily human activities will have a transformative effect on wearables and implantables. One of the main challenges in harvesting mechanical energy from human actions is to ensure that there is no effect on the body itself. For this reason, any intrusive mechanism will not have practical relevance. In this dissertation, novel non-intrusive energy harvesting technologies are investigated that can capture available energy from body, motion, and surroundings. Energy harvesting from the body is explored by developing a wrist-based thermoelectric harvester that can operate at low-temperature gradients. Energy harvesting from motion is investigated by creating a backpack and shoe sole. These devices passively store kinetic energy in a spring that is later released to a generator when it is not intrusive to the user kinematics. Lastly, energy harvesting from immediate surroundings is investigated by designing a two degree of freedom vibration absorber that is excited by electromagnetic fields found in common household appliances. These novel solutions are shown to provide consistent electrical power from wasted energy. Harvester designs are extensively modeled and optimized device architectures are manufactured and tested to quantify the relevant parameters such as output voltage and power density. / Doctor of Philosophy / Energy harvesting is the action to transform energy in the form of heat, relative motion, light, etc. into useful electrical energy. An example of an energy harvester is a solar cell which converts energy in the form of light to electricity. Our body consumes a considerable amount of energy to maintain our body temperature and achieve everyday movements, i.e., walking, jumping, etc. The purpose of this research was to fabricate, model and test wearable energy harvesters in the form of a backpack, a shoe sole, a watch, and a cantilever beam to charge mobile electronics on the go. Electrical energy is harvested from human motion by using the relative displacement between the human torso and a payload. Similarly, the ankle joint is used to produce electricity by using the relative rotation between the foot and shank. The difference in temperature between the ambient air and the human body is used to generate enough electricity to power a wrist watch. Finally, energy is harvested from everyday surroundings by using a cantilever beam which absorbs magnetic fields coming from power cords and able to power sensors.