Since its discovery in 1920, the electroencephalogram (EEG) has become a widely used tool in experimental psychology. Although originally the merits of the method were seen first of all in an improvement of medical diagnostics it was soon understood by psychologists that EEG can also be used to study psychic processes in healthy participants. This article summarizes important events in the history of EEG research that laid the ground for this development, as fast Fourier transformation to analyze the spontaneous activity and signal averaging to improve the signal-to-noise ratio. The article shows how general technological developments were the prerequisite for these methodological improvements in EEG research and how they inspired new research questions. Key discoveries that proved unequivocally that psychic processes do become manifest in EEG signals are briefly reviewed, and the emerging paradigm of cognitive psychophysiology, which is closely linked to the development of EEG research, is described.