The navigated transcranial magnetic stimulation (nTMS) technique has been developed to make the induced electric field visible to the operator when targeting anatomical loci in the cortex. Since its introduction in 1985, transcranial magnetic stimulation (TMS) has offered the scientist a unique way to stimulate the brain, but the lack of accurate knowledge about the stimulated cortical spot and the stimulation intensity at the target area has compromised reproducibility and reliability therefore limiting the clinical value of the technique. Most functional brain imaging tools indicate all brain areas that participate in a given task. Especially when there are structural and vascular changes in the brain after disease or trauma, indirect neuroimaging methods are sensitive to artifacts limiting their reliability in clinical routine work. TMS can evoke directly measurable physiological responses that produce maps from only those cortical spots that are mandatory for the observed reaction. The technological development that the stimulating electric field of the nTMS system can be calculated and visualized online for the operator has made nTMS particularly suitable for mapping the cortical motor and language areas. In this chapter, we review and explore the physical and technical background of TMS and especially the electric field-navigated TMS that has enabled clinical applications in neurosurgery.