Studies on the environmental fate of petroleum have demonstrated the nearly ubiquitous distribution of microorganisms that can metabolize hydrocarbons. The rates of degradation depend upon the concentrations of such microbes and upon the environmental characteristics of an oil-contaminated ecosystem. Given the appropriate environmental conditions, microorganisms effectively decontaminate, by their biodegradative metabolism, environments that have received petroleum pollutants. Higher-molecular-weight compounds, especially those with multiple condensed ring structures and with highly branched or substituted compounds, are relatively resistant to microbial attack. Despite the fact that a genetically engineered hydrocarbon degrader was the first organism ever patented and that seed cultures are produced by various commercial firms, enhanced biodegradation as a result of seeding generally has not been shown to be effective. Also, even though some anaerobes have now been demonstrated to be capable of hydrocarbon metabolism, hydrocarbons persist indefinitely in anoxic environments. Environmental modification, on the other hand, such as that achieved by aeration or fertilization with nitrogen and phosphorus, has been shown to enhance biodegradative removal of hydrocarbons. Having considered the various factors that influence the rates of hydrocarbon biodegradation, we are left with the question of what to do when environmental oil contamination occurs in order to minimize its persistence and thus its long-term effects. Clearly, treatment methods should enhance rather than inhibit the natural rates of oil biodegradation. In some cases, it is possible to modify environmental parameters to enhance rates of hydrocarbon biodegradation, but such methods are rarely undertaken. The translation of our scientific knowledge of hydrocarbon biodegradation into practical applications remains a major challenge. Specifically designed organisms are needed to degrade toxic aromatic components of refinery waste streams before environmental treatment. Specially designed reactors with specific microbial populations are also needed if oily sludges are to be degraded by biological means, either aerobically or anaerobically, in contained, environmentally safe reactors.