Staring into the ﬂickering ﬂames of an open ﬁre can often produce a calm state of reverie. Of course, this may be due in part to the feeling of warmth generated by the heat of the ﬁre, but it may also be related to the ﬂickering light of the ﬂames. The idea that a ﬂickering stimulus can inﬂuence behaviour is responsible for the warnings given out prior to certain television programmes, or ﬁlms that incorporate ﬂashing lights. Such warnings are given because these ﬂashing lights can directly inﬂuence the electrocortical activity of the human brain, encouraging it to mimic, or repeat the pattern of, the ﬂashes. For most people this doesn’t pose a problem, but for those suffering from epilepsy it has the potential to induce a seizure. Knowing that an external stimulus, whether light or sound, can result in what’s commonly referred to as an entrainment eﬀect on the brain has led to the development of a variety of products aimed at inducing such eﬀects to help stimulate certain behaviours. Advocates of audio-visual entrainment equipment suggest that it can produce a range of beneﬁcial eﬀects on behaviour, including reducing stress, boosting IQ, accelerating learning, enhancing creativity and improving memory.