Whereas short-term (minutes) plasticity is either presynaptic or postsynaptic, long-term (days) plasticity involves synaptic remodeling and growth, which require both presynaptic and postsynaptic mechanisms. In addition, an intermediate-term stage has been identified that lasts tens of minutes and involves the recruitment of synaptic proteins but not growth. These findings have raised two fundamental questions: How are the different stages of plasticity related, and when and how does plasticity spread from one side of the synapse to both sides? Studies of the mechanisms of short-term and intermediate-term facilitation in Aplysia have begun to answer these questions. Intermediate-term facilitation is the first stage to involve both presynaptic and postsynaptic mechanisms. Furthermore, increased spontaneous transmitter release from the presynaptic neuron during the short-term stage acts as an anterograde signal to recruit postsynaptic mechanisms of intermediate-term facilitation, which may be first steps in a cascade that can lead to synaptic growth during long-term facilitation.