Transcranial magnetic stimulation (TMS) is a noninvasive technique by which a focal electrical current is induced in the brain, by means of a rapidly changing magnetic field applied over the scalp. The main neural effect of TMS consists of direct depolarization of axons, with induction of action potentials, but short-lasting after-effects of repetitive TMS are also observed. Accordingly, the impact of TMS on behavior can be immediate (online effects) or delayed (offline effects). TMS is used in human cognitive neuroscience to establish causal relationships between brain regions and behavior. Therefore, despite significant inter-individual variability of its behavioral effects, TMS is an optimal tool for studying the hemispheric specialization of brain functions in humans.