Nine percent of the population will experience a seizure at some point in their lifetime, but only a fraction of patients will develop epilepsy, a disease of recurrent unprovoked seizures. Understanding why patients with epilepsy have an increased propensity for synchronous network activity in the brain is essential for early intervention and developing more effective therapies. In particular, mesial temporal lobe epilepsy (mTLE) is the most common form of epilepsy in adults, but it is likely that epileptogenesis begins in childhood. Pathological changes in mTLE are particularly prominent in the dentate gyrus, a critical node for controlling activity in the hippocampal formation, and one of only two regions in the mammalian brain where new neurons are born during adulthood. We use deep two-photon calcium imaging in epileptic mice to study the generation of synchronous network activity in the dentate gyrus and the role of aberrant newborn dentate granule cells in epileptogenesis.