The plasticity of the brain is an increasingly important topic for physical therapists interested in childhood development, learning, and repair following injury. The study of plasticity directly in the human nervous system presents numerous challenges, such as the ability to assess neuronal function in vivo because of physical impediments, such as the skull, skin, and dura. Transcranial magnetic stimulation (TMS), however, has become a suitable, noninvasive, and painless technique that can be applied to detect changes in cortical excitability as an indicator of neurological changes. Furthermore, repetitive trains of TMS themselves can induce plasticity. This article discusses the use of TMS to investigate and manipulate plasticity in the human nervous system.