One of the remarkable features of the mammalian brain is its capability to learn. This is particularly evident during infancy, when novel context is rapidly integrated and transformed into new memory. How early-life experiences are encoded in the developing hippocampus, a brain region central to the formation and recall of memory, has not been well understood. We know very little how neural populations in the hippocampus are activated during learning, allowing for both a generalised yet flexible encoding of information. Recent work suggests that a subset of hippocampal neurons show more rigid, stable activation patterns across the behavioural domain while others are more plastic. How their properties emerge during infancy and evolve during brain maturation is the centre of this proposal. To answer these questions, I propose to study the ensemble code in the developing mouse hippocampus. I will use a combination of calcium imaging, viral labelling, and chemogenetic techniques paired with a behavioural navigation assay to measure in vivo the activities of 100s of neurons while the animal explores known and novel environments. Specifically, it is my ambition to understand how the functional properties of rigid and plastic neurons in CA3 region emerge during development and to identify the neural circuitry encoding episodic memories in the infant and adult brain. By bridging developmental, systems, and behavioural neuroscience, the proposed research will provide highly novel contribution to our understanding of learning during brain maturation.