Increasing demand in mobile, autonomous devices has made the issue of energy harvesting a particular point of interest. Systems that can be powered up by a few hundreds of microwatts can feature their own energy extraction module, making them truly self-powered. This energy can be harvested from the close environment of the device. Particularly, piezoelectric conversion is one of the most investigated fields for ambient energy harvesting. Moreover, the extraction process can be optimized by proper treatment of the piezomaterial output voltage. This article proposes a detailed explanation of the real energy flow that lies behind several energy conversion techniques for piezoelectric energy scavenging. As well, the principles of energy harvesting using piezoelectric effect is extended to the pyroelectric effect, therefore allowing harvesting energy from temperature variation, which is one of the most common energy sources.