Achieving an appropriate balance between excitatory and inhibitory synapses as the central nervous system is wired up during development is critical for building smoothly flowing information pathways. Neuroligins are postsynaptic adhesion molecules that also play a role in synapse formation. Chih et al. (see the Perspective by Hussain and Sheng) have analyzed knockdown mutants of several neuroligin isoforms and found that various neuroligins have overlapping, but not identical, functions. Disruption of neuroligin function leads to a loss of excitatory synapses and results in a functional imbalance of excitatory and inhibitory transmission in the rodent hippocampus.