This paper addresses low-power, low-voltage electronic circuit requirements for wireless sensors with energy harvesting. The challenges of start-up for micro-controller unit (MCU)-based energy-harvesting platforms is discussed where a transient, low-voltage (20–1000 mV), low-power (<100 μW) source having a relatively high source impedance (possibly >500 Ω) is used. Efficient converter circuitry is required to transform the low-voltage output from the source to a level suitable for typical electronic devices, 1.8–5 V, and a prototype is demonstrated in the paper. Owing to the limited energy available to deliver to the storage element, the converter output voltage typically has a slow rising slew rate that can be a problem for MCUs. This necessitates a reset circuit to hold-off operation until a level high enough for reliable operation is achieved. Once operational, Maximum Power Point Tracking (MPPT) extracts peak power from the harvester while simultaneously tracking the transient nature of the source. In this low-power application, MCU programming needs to be efficient, while otherwise keeping the MCU in the lowest power standby mode possible to conserve energy. In a fully integrated design, a single MCU may be used for the sensor application, power management, power conversion, and MPPT functions.