Neuromodulation techniques such as transcranial alternating current stimulation (tACS) are a promising treatment approach for several neurological and psychiatric disorders, but suffer from variable effects due in part to their brain-state dependency. In this talk, I will show how electroencephalography (EEG) has become a useful tool to understand the immediate effects of tACS, and to implement closed-loop systems where tACS is adapted to ongoing brain oscillations in real-time. Finally, some potential clinical applications of such closed-loop approaches are discussed.