Brain-computer interfaces (BCI) have applications in function restoration, function replacement, and communication. A further potential application of BCI systems, which has been less extensively investigated, is their use for promoting plastic changes during the recovery of motor and/or sensory functions (neuromodulation). In this talk, I will focus on methods that have been developed to change the cortical excitability in humans by associating motor commands and artificial afferent volley (e.g., electrical stimulation) delivered with precise temporal delay. In such systems, the online detection of cortical activation associated to movement imagination (motor evoked cortical potential, MRCP) is equivalent to an asynchronous BCI system that triggers an external device. It has been shown that, using these systems, the excitability of the neural projections connecting the relevant brain areas to the target muscle is increased only when the afferent inflow arrives during the highest cortical activation phase (measured from MRCPs). The full asynchronous BCI system with triggering of electrical stimulation has also been recently tested in stroke patients, showing that it promotes plastic cortical changes also in this patient population, with a limited number of trials. Moreover, in stroke patients, such intervention, repeated over several days, has also been shown to improve functional recovery.