Functional neural circuit formation during postnatal development involves the elimination of early-formed redundant synapses. In the neonatal mouse cerebellum, each Purkinje cell is innervated by multiple climbing fibers (CFs). Their synaptic strengths are initially uniform but one CF strengthens relative to other CFs during the first postnatal week. Then the weaker CFs are eliminated during the second postnatal week. Projections from the retina to the lateral geniculate nucleus (LGN) undergo similar developmental changes, that is the elimination of the majority of inputs and strengthening of a few afferents that remain. Recent studies have clarified that in both cerebellum and LGN, synapse refinement consists of multiple phases and that distinct types of neural activity drive synaptic remodeling in each developmental phase.