Chlorinated solvents, such as trichloroethene (TCE) and tetrachloroethene (PCE), are released into the subsurface as dense nonaqueous phase liquids (DNAPLs), which can persist for centuries. SERDP and ESTCP efforts have encompassed a range of issues related to DNAPL in the areas of source zone depletion, fractured rock, thermal treatment, in situ chemical oxidation, nanoscale iron, bioremediation, and technology performance evaluation. Tools are being developed that enable site managers to characterize the size and longevity of the contaminated plume, apply cost-effective remediation and monitoring techniques, and predict the duration of site remediation.