Due to the high amplifications necessary for EEG recording, artefacts are also major elements of the EEG tracing. They sometimes simulate brain waves, especially in the lower frequency range, and can therefore lead to misinterpretations, particularly in anesthetized patients. It is important to recognize, mark, and eliminate them if possible. One can distinguish between biological and technical artefacts. Biological artefacts in EEG are secondary to movements, cardiovascular phenomena (pulse, ECG), or vegetative reactions (sweating) of the patient. They can be minimized by patient education, by change of recording conditions, or modification of electrode position. Electrode or cable artefacts are sometimes biological but more usually technical in origin. Especially a reduction in electrode resistence will improve the quality of the EEG recording. Technical artefacts are easily recognizable. Theoretically it is easy to isolate and eliminate the causes — 50 Hz alternating current or high frequency voltage — but in practice it is often difficult. Special attention must be paid to the recognition of artefacts in spectral analysis EEG recordings. Here ECG and pulse artefacts show up mainly as 1–2 Hz peaks; they must not be mistaken for an increase in the delta range. Like all artefacts, they are most easily recognized on the conventional EEG recording (Fig. 4). Muscular and high-frequency artefacts lie outside of the spectral range under normal registration conditions (0.5–32 Hz). Due to the mathematical-technical methodology a “reflection” of these high-frequency phenomena can appear in the frequency range of the spectral analyzer. By this means activities within the range relevant to EEG are imitated. This can be prevented by careful low-pass filtering of frequencies above 70 Hz.