Novel methods for neuronal entrainment 1–4 provide the unique opportunity to modulate perceptually relevant brain oscillations 5, 6 in a frequency-specific manner and to study their functional impact on distinct cognitive functions. Recently, evidence has emerged that tACS (transcranial alternating current stimulation) can modulate cortical oscillations 7–9. However, the study of electrophysiological effects has been hampered so far by the absence of concurrent electroencephalogram (EEG) recordings. Here, we applied 10 Hz tACS to the parieto-occipital cortex and utilized simultaneous EEG recordings to study neuronal entrainment during stimulation. We pioneer a novel approach for simultaneous tACS-EEG recordings and successfully separate stimulation artifacts from ongoing and event-related cortical activity. Our results reveal that 10 Hz tACS increases parieto-occipital alpha activity and synchronizes cortical oscillators with similar intrinsic frequencies to the entrainment frequency. Additionally, we demonstrate that tACS modulates target detection performance in a phase-dependent fashion highlighting the causal role of alpha oscillations for visual perception. •Successful simultaneous tACS-EEG recording•Entrainment of alpha oscillations by tACS•Phase-dependent modulation of target detection performance Helfrich et al. report direct electrophysiological and behavioral evidence for the entrainment of perceptually relevant cortical oscillations by transcranial alternating current stimulation, highlighting the causal role of neuronal oscillations for perception and cognition.