Zero-valent iron nanoparticle technology is quickly becoming a popular choice for remediation and treatment of a wide variety of common environmental contaminants in soil and groundwater. Over the three years, there are more than 30 completed and ongoing applications in North America and Europe. Nanoparticles have small sizes for effective in situ injection and dispersion and large surface areas and high surface reactivity for rapid contaminant transformation. Recent innovation in the technology and increasing supplies of nanoparticles have substantially reduced the cost of this technology for large scale applications. In this lecture, fundamental principles on nanoparticle synthesis and characterization will be highlighted. Applications of the iron nanoparticles for treatment of chlorinated organic solvents, organochlorine pesticides, PCBs, perchlorate, and hexavalent chromium will be presented. In addition, key issues related to field applications such as cost, fate/transport, and potential environmental impact will be discussed.