||Micro-systems with features of tiny volume and wireless communications are developed for different applications. To prolong devices’ lifetime, energy harvesting techniques which use environmental energy as the power source are proposed. Although the amount of environmental energy is theoretically infinite, it is usually unstable and the instantaneous power available is limited. To utilize the environmental energy efficiently, power management circuits have to be built to track the unstable energy status and extract maximum power from the energy transducer. In this work, we use solar energy as an example and demonstrate the design of power management circuit for micro-systems that use energy harvesting techniques. An inductor-less solar power management system was proposed and built. The system targets to operate in all lighting environments. When the light intensity is low, a charge pump is used to step up the output voltage from the photovoltaic cell. Meanwhile, charge pump switching frequency can be auto-adjusted to transfer maximum solar power to the load. Another power management circuit was proposed and designed for solar energy harvesting systems. It is based on single inductor dual-input dual-output DC-DC converter. The converter clamps photovoltaic cells to a point where maximum solar power is extracted. At the same time, it generates a stable output voltage to power the load. Also, the converter schedules energy utilization among different energy sources and consumers. The single inductor implementation can reduce the devices’ volume and cost. For devices that have very stringent volume requirement, it may not be possible to have built-in battery, and the load is powered by environmental energy only. To cater for the source characteristic, power management is executed in the load side. A charge based computation methodology was proposed where the load operation is controlled by the source energy status. Moreover, a control strategy was derived to improve system performance. For applications that are powered by environmental energy, the load circuits should utilize the energy efficiently. The energy consumption of the load should be minimized so that the environmental energy source can support more load operations. It has been demonstrated that minimum energy consumption for digital circuits occurs when operating the circuits in sub-threshold region. In this work, we also looked at the energy-efficient circuit design based on sub-threshold logics. In particular we investigated the design of proper interfaces between the sub-threshold logics and other high voltage blocks in the system. A multi-stage sub-threshold level converter was designed to up-shift the sub-threshold voltage. It functions robustly for sub-threshold input, and the energy consumption is small. The multi-stage structure helps to increase the slew rate of the output and thus reduce the short-circuit current of the logic gates driven by the level converter.