Nanoparticles (NP's) have been in the Earth since the beginning of time. It has been proved that NP's are synthesized during volcanic eruptions, forest fires, and metabolic processes of living organisms that store iron in the form of nanosized minerals. However, the impact of NP's in the ecosystem is not well understood. New developments and applications of NP's may liberate large amounts of these kinds of materials to the environment; a lot of questions are open regarding their environmental impact. This chapter is focused on iron oxide NP's developed in plants by iron metabolic routes. Additionally, a discussion of the impact of man-made iron oxide NP's on the growth of plants is described. © 2012 Nova Science Publishers, Inc. All rights reserved.