Adult-born granule cells have been implicated in cognition and mood; however, it remains unknown how these cells behave in-vivo. The authors used two-photon calcium imaging to monitor the activity of young adult-born neurons in awake behaving mice. The authors found that young adult-born neurons fire at higher rate in vivo but paradoxically exhibit less spatial tuning than their mature counterparts. When presented with different contexts, mature granule cells underwent robust remapping of their spatial representations and the few spatially tuned adult-born cells remapped to a similar degree. The authors next used optogenetic silencing to confirm the direct involvement of adult-born granule cells in context encoding and discrimination, consistent with their proposed role in pattern separation. These results provide the first in vivo characterization of adult-born granule cells and reveal their participation in the encoding of novel information.