The article by György Buzsáki and Edvard I. Moser explores the relationship between memory, navigation, and theta rhythm in the hippocampal-entorhinal system. They propose that mechanisms of memory and planning have evolved from mechanisms of navigation in the physical world, suggesting that the neuronal algorithms underlying navigation in real and mental space are fundamentally the same. The authors review experimental data supporting this hypothesis and discuss how specific firing patterns and oscillatory dynamics in the entorhinal cortex and hippocampus can support both navigation and memory. The hippocampus and entorhinal cortex are involved in guiding navigation and supporting declarative memories. The brain systems involved in navigation, including the hippocampus and entorhinal cortex, are also responsible for guiding memory and planning. The authors distinguish between two forms of declarative memory: semantic memory, which defines living things, objects, facts, and events independently of temporal context, and episodic memory, which endows individuals with the capacity to learn and recall first-person experiences in the context of both space and subjective time. The article discusses the role of grid cells, place cells, and head direction cells in the entorhinal cortex and hippocampus in representing spatial information. These cells provide a metric for neural representation of space and are active in all environments, often behaving coherently. The authors suggest that the modular representation of space during mammalian evolution laid the foundation for storing independent representations of environments and experiences. The article also explores the relationship between episodic recall and internally generated cell assembly sequences. It argues that the same mechanisms that define unique positions and their relationships in a map can be used to define or symbolize events, objects, and living things. The authors propose that theta oscillations play a crucial role in organizing neuronal assemblies and enabling episodic recall by providing temporal context and linking past and present experiences. Finally, the article addresses several challenges and questions, including the meaning of different firing patterns, the role of theta oscillations, and the mechanisms of grid map formation and segregation in the hippocampus. The authors conclude that the available experimental evidence indicates that mechanisms of entorhinal cortex-hippocampus-dependent memories evolved from mechanisms introduced to compute relationships of landmarks and track body movements in the environment.The article by György Buzsáki and Edvard I. Moser explores the relationship between memory, navigation, and theta rhythm in the hippocampal-entorhinal system. They propose that mechanisms of memory and planning have evolved from mechanisms of navigation in the physical world, suggesting that the neuronal algorithms underlying navigation in real and mental space are fundamentally the same. The authors review experimental data supporting this hypothesis and discuss how specific firing patterns and oscillatory dynamics in the entorhinal cortex and hippocampus can support both navigation and memory. The hippocampus and entorhinal cortex are involved in guiding navigation and supporting declarative memories. The brain systems involved in navigation, including the hippocampus and entorhinal cortex, are also responsible for guiding memory and planning. The authors distinguish between two forms of declarative memory: semantic memory, which defines living things, objects, facts, and events independently of temporal context, and episodic memory, which endows individuals with the capacity to learn and recall first-person experiences in the context of both space and subjective time. The article discusses the role of grid cells, place cells, and head direction cells in the entorhinal cortex and hippocampus in representing spatial information. These cells provide a metric for neural representation of space and are active in all environments, often behaving coherently. The authors suggest that the modular representation of space during mammalian evolution laid the foundation for storing independent representations of environments and experiences. The article also explores the relationship between episodic recall and internally generated cell assembly sequences. It argues that the same mechanisms that define unique positions and their relationships in a map can be used to define or symbolize events, objects, and living things. The authors propose that theta oscillations play a crucial role in organizing neuronal assemblies and enabling episodic recall by providing temporal context and linking past and present experiences. Finally, the article addresses several challenges and questions, including the meaning of different firing patterns, the role of theta oscillations, and the mechanisms of grid map formation and segregation in the hippocampus. The authors conclude that the available experimental evidence indicates that mechanisms of entorhinal cortex-hippocampus-dependent memories evolved from mechanisms introduced to compute relationships of landmarks and track body movements in the environment.