We are interested in the functional and structural organization of brain circuits that support the storage, expression and use of memories during behavior. In our research we use rodent models and focus on brain areas in the medial temporal lobe that are known to be critical for spatial and episodic memory. As animals learn novel associations or perform a memory-guided task, we observe the communication between large numbers of neurons using arrays of wires (‘tetrodes’) or specially designed neural probes. This allows us to study how populations represent the sensory world and to analyze the expression and use of memories during wakefulness or sleep. Combining the monitoring of neural activity with selective (opto)genetic or pharmacological perturbations we can tease apart the role of specific brain areas or cell types in memory processing. Finally, by reading out (‘decoding’) memories in real-time we may selectively alter and control the effect of these memories during behavior. The results of our work will not only shed light on the basic operation of memory systems in the brain, but also provide us with clues on where these systems fail under pathological conditions such as Alzheimer’s disease, sleep disorders and epilepsy.