It is common knowledge that all hydrocarbons naturally degrade in the subsurface environment, albeit at various rates under a variety of subsurface conditions. This simple fact is well documented in the literature, and has been previously mentioned in this book. Although biodegradable, petroleum hydrocarbons are the focus of cleanup efforts at thousands of sites throughout the United States, with considerable national effort being spent to accelerate cleanup of compounds which eventually degrade. Natural attenuation, as we commonly refer to this naturally occurring process, has been rapidly growing in momentum as an alternative to the high cost and limitations of engineered solutions in efforts to meet regulatory cleanup standards for groundwater. Our understanding of relative permeability, developed out of the petroleum industry during the 1930s, clearly demonstrated that not all petroleum hydrocarbons in the subsurface can be or will be mobilized, and that some appreciable amount of residual hydrocarbons will remain in the subsurface, regardless how innovative we try to be, and that such presence will likely exceed regulatory limits in many cases. This understanding is exemplified even more by the limitations of conventional pump-andtreat aquifer restoration strategies (discussed in Chapter 9), and the limitations posed by the presence of hydrocarbon-impacted low-permeability soils.