The electroencephalogram (EEG) reflects the electrical activity of the brain. Berger [2] was the first to describe the human EEG which nowadays is a standard diagnostic tool in clinical practice in particular regarding stroke, traumatic brain injury, epilepsy and sleep disturbances. In clinical settings the EEG is recorded at typically 21 electrode sites equally distributed over the scalp. Using low noise differential amplifiers, the signal of each electrode is recorded with respect to a unique reference electrode (referential recording) thereby allowing for later arbitrary re-referencing.Recent technical developments include dry electrodes as well as a wireless signal transmission from the patient's head to the computer. The spontaneous EEG as recorded under relaxed condition (resting state EEG) has become interesting as a tool to analyse neural networks.