To understand the rapid membrane dynamics or cells or synapses requires observations at high spatial and temporal resolution. “Flash-and-freeze” electron microscopy combines optogenetics with electron microscopy to capture millisecond changes in synaptic morphology during neurotransmission. These experiments indicate that membrane is internalized as quickly as 30 ms after exocytosis. Because of its rapid speed, we call this process “ultrafast endocytosis.” The internalized membrane then fuses to form a synaptic endosome which is resolved into synaptic vesicles with a t-1/2 of 5 seconds. I will discuss improvements to the instrumentation that will facilitate resolution of membrane fusion, which particularly difficult since exocytosis occurs within 500 microsceonds of stimulation.