Cerebral plasticity plays an important role in learning and in neurological recovery after nervous lesions. However, in certain nervous diseases that same plasticity may lead to nervous system malfunction. Nowadays there are new methods that seem to be able to modulate nervous system function, namely transcranial magnetic stimulation and transcranial direct current stimulation. By use of these methods it may be possible, for example, to transiently improve learning and other mental abilities in normal volunteers or to treat symptoms of maladaptive brain plasticity in various neurological and psychiatric conditions. In this article we review the history, the fundamentals and the most important uses of these techniques.