Neural oscillations are integral to various cognitive processes, including working memory, attention, and perception. Transcranial alternating current stimulation (tACS) has emerged as a noninvasive tool for investigating causal relationships between brain oscillations and cognitive functions. By targeting specific frequencies, tACS can modulate oscillatory activity, providing insights into the role of neural rhythms in cognitive performance. To understand the impact of tACS on oscillatory dynamics and cognitive processes, it is essential to combine physiological measurements such as EEG with behavioral data. High-definition tACS (HD-tACS) improves the spatial precision of stimulation, enabling more localized targeting of specific cortical areas. This protocol outlines a method for combining HD-tACS with EEG to assess changes in theta-frequency (4 Hz) oscillations in the left parietal cortex. EEG recordings are taken before and after stimulation during a 2-back working memory task to examine theta activity. This approach offers a powerful tool for exploring the neural mechanisms that underlie cognitive modulation via rhythmic brain stimulation.