Human brain activity in the gamma frequency range was shown to be a correlate of numerous cognitive functions. However, up to now there is no coherent theory predicting under which experimental conditions gamma activity would arise. One mechanism underlying all of the above cognitive functions is access to memory and may thus explain the phenomenon. We tested the hypothesis that gamma activity is evoked whenever stimuli match memory representations. EEG was recorded from 16 subjects performing a choice reaction task. Visual stimuli were either known real-world objects with a memory representation or novel configurations never seen before. Evoked gamma activity was significantly larger for items which matched memory templates. Therefore, we argue that gamma activity results from the feedback between memory and perception systems. This assumption for the first time integrates the data of many experiments and offers a unifying theory.