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Transcranial magnetic stimulation (TMS) is a technique to stimulate the human brain transcranially by a coil positioned on the surface of the head. Time-varying magnetic fields generated by the coil induce electric fields which stimulate neurons in the brain. By using a figure-eight coil, localized area of the brain can be stimulated, which enables us to study dynamic neuronal connectivity in the human brain non-invasively. The electroencephalographic (EEG) measurements just after TMS reveal the dynamic propagation of exciting fronts in the brain. This paper reviews the principles of magnetic stimulation of nerve tissues and brain, and discusses the usefulness and problems of the TMS for cognitive science and treatments of brain diseases such as depression. To obtain physiological bases for the potential treatments of brain dysfunction by repetitive TMS (rTMS), effects of rTMS on rat hippocampus are studied. The experimental results suggest that the rTMS modulates memory function, and contributes to recovery of injured neurons and acquisition of tolerance against cerebral ischemia.