A new study reveals that Nogo receptors (NgRs) have key roles in restricting synapse formation. Wills et al. showed that loss of all three NgRs in mice promoted excitatory synapse formation, whereas overexpression of NgR1 in hippocampal slices decreased synaptic density. Furthermore, they showed that NgRs seemed to specifically limit synapse addition, partly through limiting dendritic growth. Interestingly, Wills et al. found that NgR expression was downregulated by neuronal activity, possibly facilitating activity-dependent synapse formation.