Gamma rhythms have traditionally been considered as a single class of rhythms that exhibit a broad range of frequencies (~25-100 Hz). However, accumulating evidence suggests that slow (~25-55 Hz) and fast (~60-100 Hz) gamma are separate rhythms in the hippocampal network. Slow gamma rhythms synchronize hippocampal subfield CA1 with inputs from CA3, a neighboring subfield thought to store memories for previously experienced locations. Fast gamma rhythms link CA1 to medial entorhinal cortex inputs that transmit information about current spatial location. In this talk, I will present recent work supporting the hypothesis that slow and fast gamma rhythms reflect different modes of information processing in the hippocampal network. We found that slow gamma rhythms primarily occur during periods when hippocampal 'place cells' predict upcoming positions. On the other hand, fast gamma rhythms mainly occur when place cells code recently visited locations. Consistent findings were observed for individual place cells and coordinated place cell ensembles. These results suggest that alternating slow and fast gamma states correspond to distinct memory retrieval and memory encoding modes in the hippocampal network.