Synaptic plasticity is conventionally discussed in terms of its short-term and long-term aspects. Long-term plasticity (LTP), the rewiring of synaptic connections, is considered to be the basis of learning and memory. Short-term plasticity (STP) mediates more basic signal processing tasks like adaption, filtering, and working memory. Our recent work focusses on presynaptic aspects of STP, such as changes in the amount of neurotransmitter release depending on the use of a synapse. Neurotransmitter is contained in vesicles, which fuse with the plasma membrane upon arrival of a nerve impulse. We find that plasticity may not involve changes in release-willingness of fully release competent vesicles, as commonly believed, but rather reflect changes in the state of assembly of the release apparatus, which is under control of second messengers.