**1. Introduction**

Transcranial magnetic stimulation (TMS) is an experimental tool that allows researchers to noninvasively explore various neural processes and measure a variety of cortical phenomena and different timescales. The most important aspect of TMS is its ability to directly stimulate the cortical neurons, generating action potentials, without much effect on intervening tissue. This property can be leveraged to provide insight into the pathophysiology of various neuropsychiatric disorders. Using multiple patterns of stimulations (single, paired, or repetitive), different neurophysiological parameters can be elicited. In this article, we review the role of TMS as a tool to study motor neurophysiology of major neuropsychiatric disorders. TMS-related parameters reflect underlying cortical excitability changes during any brain motor action. New findings of motor system abnormality through TMS parameters have provided new insight into the pathophysiology of neuropsychiatric disorder.
