A maximum-power-point-tracking DC-DC boost converter to harvest energy from sub- μW power sources is presented. For available input-power levels below 1 μW, voltage boosting is achieved by operating all circuits in the sub-threshold region, and by switching the DC-DC converter at tens of Hz, thereby reducing switching losses. The paper further explores the possibility of energizing the DC-DC inductor for an optimum duration, such that switching and resistive losses are minimized. The sub- μW energy harvesting circuit uses an area of 0.2 mm² on a standard 180 nm CMOS process, and utilizes an auxiliary voltage source for start-up. The designed and fabricated system is more than 50% efficient when the available power is greater than 2 μW. The circuit can harvest energy whenever the available power is more than 0.3 μW. Efficiency at 0.3 μW is 25%, at 0.5 μW is 37% and at 1 μW is 48%. The complete IC consumes 50 nA for internal operations and the input voltage can be as low as 70 mV.