Contamination of groundwater by chemicals used in industrial, commercial, and other applications continues to pose significant threats to human health and the environment. Examples of compounds of concern include chlorinated solvents (e.g., trichloroethene, tetrachloroethene, carbon tetrachloride), 1,4-dioxane, methyl tertiary-butyl ether (MTBE), and perchlorate. Extensive dissolved-phase groundwater contaminant plumes typically form at sites contaminated by these compounds because of their relatively high aqueous solubilities (in comparison to regulatory standards), limited retardation, and generally low (or very site dependent) transformation potential. In many cases, the plumes are hundreds of meters to several kilometers long. It is now recognized that most sites with large groundwater contaminant plumes will require many decades before cleanup can be achieved under current methods and standards. The primary transport-related factors that contribute to plume persistence will be discussed and illustrated via case study, including uncontrolled source zones, dispersed reservoirs of dissolved contaminant (present in lower-permeability zones), and hydraulic-related factors such as nonoptimal remedial well-field performance. Potential remediation alternatives will also be discussed.