The nature of the subsurface is important to humans for a number of reasons. We build our houses and infrastructure on the surface thus the structural integrity of the subsurface is important. A substantial percentage of our drinking water comes from groundwater aquifers and contamination of these aquifers by effluent or leachate from landfill sites is a major concern for many individuals, states and even countries. In addition, most of our energy resources and virtually all our mineral and aggregate resources are obtained from the subsurface. Geophysical techniques provide the tools we require in order to obtain useful information about the subsurface. Geophysics is a science that transcends the usual disciplinary boundaries and thus this book will also be beneficial for engineers, planners, archaeologists, hydrologists, geographers and environmental scientists. Aims and scope of the book are: to explain the theoretical basis behind the main geophysical techniques used in environmental investigations; to discuss the equipment used in such investigations and to provide case studies of the use of the techniques.