Remediation of soils and sediments contaminated with chlorinated solvents and heavy metals has been a major environmental challenge for decades. Yet, cost-effective in situ remediation technology remains lacking. While zero-valent iron (ZVI) nanoparticles have been found effective in reducing various chlorinated hydrocarbons, rapid agglomeration of the particles rendered the particles undeliverable in soils. To address this issue, we developed a particle stabilization strategy using starch or carboxymethyl cellulose (CMC) as a stabilizer. We found that the use of the stabilizers can facilitate controlling the size, delivery and transport of the nanoparticles and resulted faster reaction rate. The stabilized ZVI nanoparticles can be readily delivered to the targeted contaminated zones, and can in situ effectively destroy chlorinated solvents such as trichloroethylene (TCE). Bench- and field scale experimental data showed that the stabilized ZVI nanoparticles can in situ completely and rapidly dechlorinate TCE in water and soils. Field tests also indicated that the application of stabilized ZVI nanoparticles can boost long-term biodegradation of chlorinated solvents. Based on the stabilized ZVI nanoparticles, we also developed an technology for in situ reductive immobilization of Cr(VI) in soils and groundwater. When a Cr(VI)-laden soil column was treated with 5.7 bed volumes of 0.06 g/L of the nanoparticles at pH 5.60, only 4.9% of the total Cr was eluted compared to 12% for untreated soil under otherwise identical conditions. Moreover, the ZVI treatment reduced the TCLP leachability of Cr in the soil by 90%, and the California WET leachability by 76%. The stabilized nanoparticles may offer a powerful alternative for in situ dechlorination or in situ reductive immobilization of redox-sensitive heavy metals.