This book is the first comprehensive review of the subsurface monitoring theory and practice. It presents all aspects of a subsurface monitoring system SM-NI to be organized in impacted areas of nuclear industry enterprises. The content of the book covers the whole set of the SM-NI projecting and implementation issues - from the theoretical and regulatory framework, through the description of the sources of impact on geological environment to examples of use the SM-NI to solve environmental problems on the main types of environmentally significant nuclear industry enterprises. Development of the SM-NI was based on long-term research studies including modeling of pollutants transport in the geological environment. The book is intended for a wide range of nuclear industry employees and specialists in environment protection and radiation safety of nuclear industry enterprises. It might be useful for ecologists, students and postgraduates concerned about environment protection in the field of nuclear technology.