Electroencephalography is a diagnostic technology: the recording of brainwaves, i.e. the rhythmic fluctuations in the electrical activity of the brain’s neurons. As diagnostic practice, electroencephalography was quickly implemented for testing neuropsychiatric conditions. By the 1930s it had already proved tremendously important for characterizing and differentiating epileptic diseases. Before more advanced imaging technologies became available, the method was also helpful for localizing brain tumors and for investigating disturbances of consciousness. For some of these conditions, electroence-phalography is still in use today, for example for the determination of brain death. An EEG, the record of the brain’s electrical activity, is a medical and legal requirement for organ transplantation. For other conditions, such as the diagnosis of brain tumors, for example, it has been replaced by other technologies. What can and shall a cultural history of electroencephalography deliver in addition to understanding its use as a particular medical and diagnostic technology? Like other tools, the EEG went through various manifestations and transformations before it was tamed to a diagnostic tool of specified significance. From its very beginning electroencephalography revealed, for example, surprising relations between the physical characteristics of brain activity and mental conditions. The EEG appeared to be situated at the brink of mind and brain, providing promising access to their articulation. The recorded traces showed the undulations of electric activity, but their significance seemed to reach beyond the realm of mere physics into mental life: Intellectual activity replaced the regular oscillations of the idle brain with smaller and more irregular waves. In addition, the distribution of particular wave patterns appeared to correlate in initial findings with personality profiles. The EEG thus provides ample opportunities for a cultural history of brain and mind during much of the twentieth century. And this book shows how this came about.