To date the circuit mechanisms underlying memory formation and retrieval remain largely unknown. Engram cells are defined as the cells responsible for encoding a memory. The expression of the immediate early gene (IEG) cfos has been most directly linked to the formation and maintenance of memories. Sharp wave ripples (SWRs) on the other hand are high-frequency oscillations which have also been shown to be critical for the consolidation of memories and the abolishment of hippocampal SWRs interferes with learning. It is howeverunclear how these two mechanisms of memory formation are linked. Using genetic tagging of cfos-expressing neurons in freely behaving mice we characterize the link of engram cells and SWRs to improve our understanding of how cellular and circuit mechanisms are connected. We identify engram neurons using opto-tagging experiments and reveal new insights about their in vivo activity profile.