With the advent of diverse molecular, cellular and behavioral tools, it became possible to better understand how memory is acquired and stored precisely in molecular and cellular terms. Combined with the concept of engram, the field of learning and memory has drawn much attention in neuroscience. However, it is not clear how much engram synapses between engram cells in different brain regions contribute to the memory formation. Therefore, we have asked how memory formation strengthens engram synapses between engram cells. In this talk, I will present our recent structural and functional approaches to reveal the dynamic nature of the engram synapses.