Abstract: A prevailing notion in memory research is that the persistence of a memory depends on the stability of the neural codes established during learning. However, recent studies have shown that hippocampal place codes and cortical sensory codes gradually change (or drift) over time, challenging the idea that stable codes are required for stable memories. I will present results from experiments using longitudinal optical imaging to study hippocampal and entorhinal codes for long-term spatial memory. Our findings indicate that spatial memory content can be reliably preserved over weeks, even without stable hippocampal representations. Like hippocampal place codes, entorhinal grid codes also drift over time. Unlike place cells, however, grid cells (within a module) drift coherently, reflecting shifts in their anchoring to the external reference frame. I will discuss how these findings inform current theories of memory and spatial cognition.