The sensing of soil gas for detection and mapping of volatile organics is a relatively new technique that deserves greater attention. Soil organic vapor monitoring has been shown to be a cost effective means of delineating the size and movement of organic contaminants in the subsurface. It has also been shown to provide immediate information of the lateral extent of soil and ground-water contamination and to minimize and more accurately predict the number and location of conventional monitoring wells that must be drilled. Literature on the technique for mapping soil and ground-water contamination has been increasing, but comprehensive reviews of the method have been limited. This document is meant to be a primer on the current state-of-the-art of soil gas sensing as it relates to the detection of subsurface organic contaminants. It is hoped that such a document will better assist all those individuals who are faced with assessing the extent of contamination of our soil and ground water. The document begins by outlining many of the parameters (water solubility, microbial influence, ground-water flow, etc.) that must be considered by the scientist before utilizing soil gas sensors in a field monitoring program. Next, the complex soil, air, water, and hydrocarbon system is addressed with an overview of the important processes involved in the transport and fate of organic contaminants in the soil. Additional sections address the correct sampling and analytical methodologies for monitoring volatile organics in the subsurface, covering such sampling methods as headspace, ground probe, flux chamber and passive sampling techniques. Analytical methods include organic vapor analyzers (OVAs) and gas chromatographs with a variety of detectors. A statistical treatment of soil organic vapor measurements is also included to ensure that soil organic vapor monitoring programs address the requirement for data precision. The statistical section also gives greater insight into understanding the spatial patterns of soil organic vapor measurements. Finally, case studies are included to give the unfamiliar reader examples of the design, procedures, and results of soil organic vapor monitoring programs that have been successful in delineating the size and lateral extent of subsurface organic contaminants.