The ability of dendritic spines to change shape in response to synaptic activity is crucial for synaptic plasticity. This motility is regulated by αN-catenin, report Abe et al. on page 357. Overexpression of αN-catenin (green; red is PSD95) stabilized spines in cultured neurons, reducing turnover and thereby increasing their number. Lack of αN-catenin increased spine motility, even at established synaptic contacts. Spine αN-catenin was regulated by synaptic activity: blocking activity with tetrodotoxin reduced αN-catenin staining (and increased spine motility), whereas blocking inhibitory neurotransmission increased αN-catenin.The catenins link cadherin cell adhesion molecules to the cytoskeleton, so αN-catenin is well placed to regulate spine dynamics.