Recent in vivo and in vitro studies have used the rodent olivocerebellar path to examine developmental synaptogenesis, in particular the mechanisms which permit the simultaneous stabilisation of some synapses and elimination of others. They reveal that while the formation of synapses can take place throughout life, elimination of supernumerary connections is limited to a critical period during development. Synapse elimination involves the strengthening of appropriate synapses and weakening of others, which are subsequently removed. This process is partly dependent on activity and post-synaptic signalling cascades and irreversibly changes each synaptic partner, leaving a trace of previous connectivity. Here we discuss the nature of this << trace >>, which may be a molecule that labels and protects those synapses to be retained, and its functional significance in maintaining the specificity of neuronal circuits needed for coordinate behaviour. double dagger.