Ground-water extraction and treatment, or pump and treat, is the most commonly used technology for remediating contaminated ground water at hazardous waste sites in the United States. There are major limitations to using this technology for restoration of aquifers to drinking-water quality in a reasonable time frame. The major limitations to pump and treat technology, which are connected with the difficulty in extracting of contaminants from the subsurface, can be explained in terms of the basic processes controlling subsurface contaminant transport and fate. The same processes that limit the effectiveness of pump and treat limit most other aquifer remediation technologies, as well. It is important to understand and account for these processes when designing aquifer remediation projects. Research is being carried out by the United States Environmental Protection Agency and other organizations to reduce some of the limitations and improve the efficiency of pump and treat.