Nanotechnology is an emerging engineering field with multiple applications in industry, medicine and in environmental engineering. The use of nanomaterials may help in the remediation of contaminated sites, soil and groundwater, alone or in combination with other remediation technologies. Nanomaterials may stabilize metals or inorganic anions and eliminate hydrophobic and recalcitrant organics that are difficult to remove with the available technologies. At the nanoscale, materials show new and unexpected properties, different than those at the macroscale. Thus, for instance, iron nanoparticles show a great reactivity and reduction capacity that can be used in the dehalogenation of recalcitrant organic contaminants. In this chapter, the properties of nanoparticles and the possibilities of nanotechnology in the remediation of contaminated sites are reviewed and discussed. Special attention is paid to nanoscale iron particles due to their wide range of application in environmental engineering, but the applications of other types of nanoparticles is also discussed. Finally, the application of nanotechnology in remediation at the field scale is reviewed, analyzing the cost and the possible risks associated with the release of nanoparticles to the environment.