This chapter presents a literature review about the various ambient energy sources that can be harvested and the description of some related systems. As such, this chapter will provide a brief overview about each source and describe some systems that use that same energy source. This will be extended on to the domain of the wireless sensor networks and the aspects related to it, being presented some examples of energy harvesting powered WSN in different environments. A brief description of what is expected from each network and how it succeeds in harvesting the energy that enables it to work will receive a particular focus. Since light is the energy source being harvested, in order to power the system described in this book, some more attention will be dedicated to the analysis of this source.