Despite its clinical relevance, direct electrical stimulation (DES) of the human brain is surprisingly poorly understood. Although we understand several aspects of electrical stimulation at the cellular level, surface DES evokes a complex summation effect in a large volume of brain tissue, and the effect is difficult to predict as it depends on many local and remote physiological and morphological factors. The complex stimulation effects are reflected in the heterogeneity of behavioural effects that are induced by DES, which range from evocation to inhibition of responses - sometimes even when DES is applied at the same cortical site. Thus, it is a misconception that DES - in contrast to other neuroscience techniques - allows us to draw unequivocal conclusions about the role of stimulated brain areas.