The consolidation of memory contents in the brain is dependent on sleep-related processes and their associated oscillatory activities. Previous research has indicated a relationship between theta activity and memory consolidation in non-rapid eye movement (NREM) sleep. Yet, it is not known whether theta oscillations are causally linked to memory consolidation, since studies to date have only been able to show correlative associations. As part of a larger project investigating the functional relevance of theta in encoding, reactivation, and retrieval processes, the present study was conducted to investigate the functional relevance of theta oscillations in memory reactivation by using transcranial alternating current stimulation (tACS). Participants were recruited from a student-cohort of the University of Fribourg (Switzerland). In the experiment, the participants learned Dutch-German word-pairs before going to sleep. When they entered NREM sleep, theta-tACS (4-8 Hz) was applied over bilateral fronto-parietal cortices during the reactivation of Dutch cues. In order to demonstrate that the hypothesized effect is specific to the theta frequency band, we compared it to 23 Hz tACS. We hypothesized improved memory recall for those items which were reactivated during sleep altogether with theta stimulation, compared to those with beta stimulation and with words that were not reactivated (uncued). Overall, the results presented in the thesis did not find a significant effect for theta-tACS on memory performance as compared to beta stimulation and uncued words. Hence, the results from this study cannot explain the functional role of theta in memory consolidation processes, although it might inform future research directions.