Since the initial observations by Hans Berger in the 1920s, electroencephalography (EEG) has opened a window into studying otherwise unobservable phenomena of the brain. But how do these electrical traces come to be and moreover, what do they mean? In this chapter, we go through the physiological origins of the EEG trace recorded from the scalp, showcase the different signals that are commonly derived from it, and explore how they are used in different contexts. From sleep stages to cognitive processes, epilepsy or psychiatric conditions, the EEG has been a rich source of information on the state of the brain. Understanding these dynamics can enrich the interpretation of EEG observations, fuel new ideas, and facilitate communication with other neuroscientists.