**6. Summary**

Vadose zone contamination by NAPL hydrocarbons through either natural or industrial processes represents a worldwide concern due to its potential hazard to the environment and health impact to biological receptors. Several scientific and engineering remediation strategies have been researched, developed, field-tested, and subsequently implemented to restore these contaminated sites. For a successful risk management of a contaminated vadose, the contamination must be prevented from spreading and be removed as economically as possible in a time-efficient and practical method. In these capacities, *ex situ* surfactant enhanced bioremediation has been attracting increasing attentions in recent years. Biosurfactants and chemically synthesized surfactants are relatively low-cost production industrial process. They have been playing an increasing and pivotal role in *ex situ* remediation of contaminated soil due to their unique desorption function capability, strong solubilizing power of hydrophobic organic chemicals, and considerable enhancement of contaminants bioavailability. Several critical issues have, however, to be vigorously researched. However, the data current available indicate some research gap areas. Therefore, a concerted research endeavor is currently needed to better elucidate the fate and behavior of synthetic surfactants in natural ecosystems, mechanism of soil biota toxicity and regulation, hysteresis effect on treated soil properties, metabolites production during biodegradation, soil hydrophobicity increase, synergistic properties of mixed surfactants, combined use of surfactants with additives on enhancing bioreactors performance. Furthermore, the prospects of future development and industrial production of mixed surfactant systems combined with low CMC are very promising alternatives to either biosurfactants or chemically synthesized surfactants. This new generation of surfactants will offer the possibility of removing the large-scale remediation impediments associated with current *ex situ* surfactant-based soil remediation technology.
