Synaptic transmission is initiated when Ca²⁺ influx triggers the exocytosis of synaptic vesicles. Repetitive high-frequency stimulation of the nerve terminal often leads to rapid run-down of synaptic output due to the depletion of a limited pool of readily releasable vesicles. The measurement of the size, fusion rate, and replenishment of this vesicle pool has become an important tool for understanding short-term synaptic plasticity. In addition, several other vesicle pools have been described, indicating that vesicles go through a number of discrete states characterized by distinct properties. The molecular background for these pools is an important topic of current research.