As early as 1934 evidence appeared in animals that waves of activity do not always remain stationary but can actually spread over the whole cortex. In the next year some of the earliest studies on the EEG of man showed phase changes of alpha activity that could account for the TW, which later was described when apparatus was built to better view it, especially in the 1950s. The TW has been described mainly as alpha activity that appears to travel both in abnormal and also normal conditions, including the resting state. The phenomenon seems to be enhanced with either external stimuli or endogenous emotional states, which increase phase changes on different brain areas. The travel has been described in all directions from the frontal to the occipital pole, and early work suggests that the posterior-anterior direction may be more often found in abnormal mental states. The speed of travelling over the scalp has varied usually from 1-20 m/sec, but generally has been reported around 5 m/sec. Most investigators have reported that the positive phase is the one which most clearly travels. In exploring the phenomenon of the travelling wave, it is clear that maximal positivity and negativity of the alpha is not always on the occipital regions, as many clinical studies would imply; instead, the fronto-central areas in particular are often the focus of maximal alpha.(ABSTRACT TRUNCATED AT 250 WORDS)