Iron nanoparticles are increasingly being applied in site remediation and hazardous waste treatment. Nearly a decade after it was first proposed in 1996, the iron nanoparticle technology is at a critical stage of its developmental process. Significant research innovations have been made in terms of synthetic methods, surface property modification, and enhancement for field delivery and reactions. Extensive laboratory studies have demonstrated that nanoscale iron particles are effective for the treatment of a wide array of common groundwater contaminants such as chlorinated organic solvents, organochlorine pesticides, polychlorinated biphenyls (PCBs), organic dyes, and various inorganic compounds. Several field tests have also demonstrated the promising prospective for in situ remediation. Nonetheless, there are still considerable knowledge gaps on many fundamental scientific issues (e.g., fate, transport, and environmental impact) and economic hurdles, which could determine the acceptance of the technology within the academic community as well as by regulators and the private sector. An overview of the iron nanoparticle technology is provided in this article, beginning with a description of the process fundamentals. This is followed by a discussion of the synthetic schemes for the nanoparticle types developed at Lehigh University. Next, a summary of the major research findings is provided, highlighting the key characteristics and remediation-related advantages of the iron nanoparticle technology versus the granular/microscale iron technology. A discussion of challenges related to its future directions and environmental impact is presented. © 2006 Wiley Periodicals, Inc.