While bioremediation has many advantages, it is a site specific process and successful biological treatment of contaminated soils presents a challenge to environmental scientists and engineers for reasons including: (a) heterogeneity of the contaminants, for example, the contaminants can be found as solids, liquid, gases, free or tightly bound to the particulate matter; (b) extreme concentrations of hydrocarbons, for example, the presence of high concentrations of hydrocarbons can be inhibitory or toxic to the microorganisms while extremely low concentrations may not be adequate to support microbial activities; (c) variable site environmental conditions such as soil type and depth and soil microorganisms as well as physical conditions such as pH, temperature, oxygen availability, redox potential, moisture content and substrate bioavailability. These conditions can substantially affect the microbial growth and biodegradation of organic contaminants; and (d) bioremediation is also a slow process and subject to regulatory constraints which influence its selection as the clean-up technology, particularly with respect to the required clean-up standards and the pressure for immediate site spill or clean-up mandated by public concern, which do not allow enough time for process optimization.