Transcranial magnetic stimulation is a non-invasive tool for the electrical stimulation of the cerebral cortex, and for evaluation of the excitability of the motor cortex and pathways. Transcranial magnetic stimulation can be applied as single pulses of stimulation, pairs of stimuli separated by variable intervals to the same or different brain areas, or as trains of repetitive stimuli at various frequencies. Single stimuli can depolarize neurons and evoke measurable effects. Series of stimuli (repetitive transcranial magnetic stimulation) can modify the excitability of the cerebral cortex at the stimulated site and also at remote areas along functional anatomical connections. TMS might provide novel insights into the pathophysiology of the neural circuitry underlying neurological and psychiatric disorders, but more work is needed to establish the role of transcranial magnetic stimulation in clinical neurology.