Chapter 9 introduces transcranial magnetic stimulation (TMS), a neurostimulation technique that uses rapidly changing magnetic fields to induce electric currents in targeted brain regions. Beginning with its historical roots in 19th-century electromagnetic experiments and evolving through Anthony Barker’s groundbreaking 1985 demonstration, TMS has become a critical tool for establishing causal relationships between brain activity and behavior. Unlike neuroimaging methods that only observe brain activity, TMS can temporarily interrupt or enhance neural processing, enabling researchers to create “virtual lesions” and directly test hypotheses about regional brain function. The chapter examines TMS delivery methods, single-pulse, paired-pulse, and repetitive stimulation, and their differential effects on cortical excitability. It details four primary research applications: virtual lesions for establishing causality, chronometry for determining processing timelines, mapping functional connectivity between brain regions, and tracking neuroplasticity. Clinical applications are discussed, particularly for treating depression and presurgical mapping. The chapter also addresses practical aspects of TMS implementation, localization techniques, and safety considerations, concluding with a brief overview of transcranial direct current stimulation (tDCS) as a milder alternative stimulation approach.