The abbreviation EEG stands for electroencephalogram. This examination is a recording of weak electrical signals derived from the surface of the skull. The brain is subject to electrical activity and these signals can be recorded, in addition to providing insight into the processing that takes place in the brain. Thus, from the electrical frequencies on the scalp, one can infer the brain's internal activities. However, the EEG quickly reaches its limits, so that certain results can only be evaluated on the basis of experience; moreover, the measurement results are quite susceptible to interference. It is proven that hypnosis affects the brain waves in the EEG. Here, differences between sleep, the deep state of relaxation and the hypnotic trance are particularly evident.