Neurogenesis occurs in the dentate gyrus of the adult mammalian hippocampus and it is involved in learning, memory, and spatial encoding. Adult-born granule cells (aGCs) are continuously incorporated into the dentate gyrus in a highly regulated process, projecting to interneurons and pyramidal cells in CA3. Electron microscopy studies have shown that developing aGCs first establish their presynaptic terminals (mossy fiber boutons, MFBs) onto thorny excrescences of pyramidal cells already occupied by preexisting connection. As aGCs mature, however, these connections are refined into a single terminal per spine. These findings have led to the hypothesis that integration of aGCs is governed by activity-dependent synaptic competition. To test this hypothesis, chemogenetic or optogenetic actuators were expressed in defined cohorts of aGCs allowing manipulation of their activity in vivo. Newly generated aGCs with enhanced activity displayed a higher MFB density compared to controls. MFBs from preexisting mature neurons located in close proximity to active aGC terminals displayed a reduced size, which suggest a diminished synaptic strength. In contrast, MFBs neighboring inactive aGCs were unchanged. This provide direct evidence that neuronal activity promotes the establishment of new synapses at the expense of preexisting ones. Ongoing experiments will define the critical windows for synaptic competition, dissect underlying mechanisms, and determine the functional implications