The resting state EEG (rsEEG) recorded in the usual conditions of clinical routine in a hospital is a powerful tool in the diagnosis process of several pathologies, such as epilepsy, conscious state alterations, encephalopathies, neurologic focality, stroke, coma evaluation, or psychiatric illness. The biological foundations of EEG are a homeostatic group of neural circuits genetically regulated and the alterations obtained in the EEG can reflect, in more or less degree, the pathophysiological alteration. We describe the anatomical basis of EEG and its neural sources. Later, we shall describe the main findings obtained in a healthy adult human EEG. We describe in some detail the main waveforms obtained in the EEG, divided into different bands, namely delta (δ, 1–4 Hz), theta (θ, 4–8 Hz), alpha (α, 8–13 Hz), and beta bands (β, 13–30 Hz).