Synaptic endocytosis retrieves exocytosed vesicles and maintains synaptic transmission which is essential to neural circuit functions. Accumulated studies suggest that calcium influx triggers synaptic vesicle endocytosis, which must undergo membrane pit formation and fission of the pit’s neck to generate vesicles. However, the calcium sensor that links calcium to endocytic machinery remains not well understood; whether pit formation involves clathrin remains debated, what mechanism controls the endocytic vesicle size remains not well understood either; the mechanism that couples exo- to endocytosis remains not fully understood either. My thesis work aims at improving our understanding of each of these questions. I studied endocytosis using a combination of techniques, including gene knockout, gene knockdown, fluorescence imaging, electron microscopy, and molecular biology techniques. I identified the calcium sensors that link calcium influx to endocytosis – the protein kinase C α and β isoforms and calmodulin. I found that clathrin is involved in mediating endocytosis at synapses, which may clarify the doubts on whether clathrin is indispensable for synaptic vesicle endocytosis. I found that dynamin is crucial not only for fission as generally thought, but also for controlling the vesicle size at hippocampal synapses, which enhances our understanding on how vesicle size is regulated at synapses. I found that NSF, which disassembles the SNARE complex, is crucial for mediating synaptic vesicle endocytosis, which enhance our understanding of the mechanisms that couple exo- to endocytosis. Consequently, In summary, I identified endocytosis calcium sensor as protein kinase C (α and β isoforms) and calmodulin; found clathrin in playing a role in pit formation, discovered a novel function of dynamin in controlling vesicle size, and reveal NSF in coupling exo- to endocytosis. These findings contribute to better understanding regulation of endocytosis at synapses.