Extremely low frequency (ELF) is the ITU designation for electromagnetic radiation (radio waves) with frequencies from 3 to 30 Hz, and corresponding wavelengths from 100,000 to 10,000 kilometers. In atmospheric science, an alternative definition is usually given, from 3 Hz to 3 kHz. In the related magnetosphere science, the lower frequency electromagnetic oscillations (pulsations occurring below ~3 Hz) are considered to lie in the ULF range, which is thus also defined differently from the ITU radio bands. The major emphasis of this book is on physical mechanisms and sources of the ULF/ELF natural electromagnetic fields noises. One of the challenges of this research is to fully understand electromagnetic effects and physical processes in the rocks deep in the Earths crust.