This text is an introduction to the problem of low frequency electromagnetic monitoring, with particular emphasis on VLF band. All related issues have been addressed: from historical outlook to numerical analysis, from seismology to geotechnics, electronics, information technology. The approach is informative and didactic. This interdisciplinary approach does not imply a problem of skills to the reader, and my goal has been to complete each specific cultural background. Geophysics is a topic where different training paths converge, usually geological or physical. In this case, addressing also technical aspectcts, it is not really possible to setup a guide for a specific professional figure. For this reason, while some topics will be new, others instead will inevitably seem trivial or superficially addressed. The reality is that every topic, as far as possible, has been treated on the same level. When necessary, an appendix to the text will offer further specific insights from time to time.