Psychiatric disorders have become a major global health challenge, and traditional treatments often have limitations. Transcranial magnetic stimulation (TMS), a noninvasive neuromodulation therapy, has shown significant potential in clinical settings. This chapter discusses the fundamental principles of TMS, common modalities such as repetitive TMS (rTMS), intermittent theta burst stimulation (iTBS), and deep TMS, as well as treatment procedures, precautions, and underlying mechanisms. TMS modulates neuronal excitability, synaptic plasticity, and the functional connectivity of brain circuits, with variations in stimulation frequency, intensity, and pulse parameters significantly influencing its outcomes. While TMS has proven effective in treating resistant psychiatric conditions like depression with fewer side effects, further research is needed to optimize its mechanisms, personalize treatment protocols, and enhance equipment. This chapter aims to provide psychiatrists, neuroscientists, and other medical professionals with a comprehensive understanding of TMS, promoting its advancement and broader application in psychiatric treatment.