In recent years, we have witnessed the emergence of new techniques for studying the mechanisms that underlie perceptual and cognitive function in the human brain. An important contribution has come from the introduction of non-invasive brain stimulation (NIBS). The development of NIBS techniques to study perception and cognition constitutes a significant breakthrough in our understanding of the changes in the brain that may account for behavioral plasticity. NIBS approaches aim to induce changes in the activity of the brain, which can lead to alterations in the performance of a wide range of behavioral tasks (Sandrini et al. 2011). NIBS techniques that are used to modulate cortical activity include transcranial magnetic stimulation (TMS) (see Chapter 1) and transcranial electric stimulation (tES) (see Chapter 4). TMS and tES can transiently influence behavior by altering neuronal activity through different mechanisms, which may have facilitative or inhibitory effects. The relevance that NIBS has recently gained in the field of cognitive neuroscience is mainly derived from its ability to transiently probe the functions of the stimulated cortical area/network by changing behavior. These behavioral changes can sometimes be related to its effects on modulating cortical excitability, but the explanatory route is not always direct. This opportunity to probe and modulate functional brain mechanisms opens up new possibilities for basic cognitive neuroscience and in the field of cognitive rehabilitation in directing adaptive cognitive plasticity in pathological conditions.