The Exxon Valdez oil spill (EVOS) has been the most studied case regarding the applicability of bioremediation. Fertilizer addition was used in Prince William Sound from 1989 through 1991. The application of bioremediation to EVOS followed extensive laboratory and field demonstration tests to determine safety and efficacy.  Field tests showed, with high statistical significance, that the rate of oil degradation was a function of the ratio of nitrogen/oil, the non-polar hydrocarbon fraction remaining, and time. Addition of fertilizer increased the rate of polycyclic-aromatic- hydrocarbon (PAH) degradation in the relatively undegraded oil found on the shorelines at this time by a factor of 2. Extensive surveys have shown that most of the oil has disappeared due to natural weathering, but that some patches of subsurface oil (SSO) persist in the intertidal zone where they are protected and sequestered by surface boulder/large cobble armor and underlying bedrock or peat layers. Even where SSO patches remain, the vast majority of these residues are highly weathered, exhibit negligible bioavailability, and are widely separated from the important biological areas and wildlife foraging in the lower intertidal zone. Very few samples have less than 99% loss of aliphatic hydrocarbons and 70% loss of polycyclic aromatics. Nutrients are available in the surrounding pore water, some of the nitrogen coming from nitrogen-fixing Frankia associated with Alder trees that line the shorelines. The limitation to further oil bioegradation is the lack of water flow through the thin layers of sequestered oil, While the residual oil could be further biodegraded if the oil was mobilized there appears to be little benefit to trying to remove oil that is sequestered and not bioavailable.  Thus there are times when bioremediation is appropriate and others when it is not.