Networks of autonomous microsensors promise to revolutionize the way people interact with electronics, enabling ambient intelligence. In this paper we investigate different energy scavenging options in real applications to find out which devices are more suitable to power a microsensor having a volume of only 100 mm3. Photovoltaic cells are the most promising power generators when a power level of 100 µW is required. If the power consumption can be reduced to about 10 µW, MEMS-based vibration-driven scavengers and thermoelectric generators can also be used. The paper presents as well a power management architecture that enables the use of a scavenger as power source, and discusses some consequences of the limited available volume on power electronics.