Cranial Electrotherapy Stimulation (a.k.a. cranial-electro stimulation and transcranial electrotherapy) is the application of small electrical alternating currents (e.g.1mA) to the brain, externally, via electrodes on the scalp. It has been in use since the late 1700s.

It is (or was) used to treat a wide range of disorders, including insomnia, depression, anxiety etc etc.

The mechanism of action is unknown - and several recent studies have found that it's ineffective for almost all of the conditions which it has traditionally been used to treat. It may have some benefit for treating Major Depressive Disorderplugin-autotooltip__plain plugin-autotooltip_bigMajor Depressive Disorder

Major Depressive Disorder (MDD), also known simply as 'depression', is a mental disorder characterized by at least two weeks of pervasive low mood.

Current estimates are that severe depression affects around 2% of the global population at any given time, currently more than 150 million. The majority of cases don't appear to have an obvious personal and/or societal cause. but clear evidence for its effectiveness is lacking (ref. Cochrane Database of Systematic Reviews )

Further reading see Wikipedia

Note: A newer version of the treatment, first tested in humans in 2008, is called Transcranial Random Noise Stimulation and uses randomised noise signals to the brain instead of sine-wave alternating current. Its mechanism of action is also unknown, and it has not yet found clinical application.

Also see : Electroconvulsive therapy (ECT)plugin-autotooltip__plain plugin-autotooltip_bigElectroconvulsive therapy (ECT)

Electroconvulsive therapy (ECT), formerly known as electroshock therapy, is a psychiatric treatment in which strong seizures are electrically induced in patients, After treatment, by an unknown mechanism, it provides relief from some severe mental disorders - especiallyand Deep Brain Stimulationplugin-autotooltip__plain plugin-autotooltip_bigDeep Brain Stimulation

Deep Brain Stimulation (DBS) is therapeutic neurosurgical procedure used in the treatment of Parkinson's disease and Tourette syndrome

See Wikipedia

It was introduced in 1997, and involves direct electrical stimulation (via implanted wires) of various brain regions. Because of the surgical risks, it's usually only used for severely affected patients, when drug treatment is not proving effective.