FM1-43 and similar styryl dyes have proven useful as probes for membrane trafficking because they reversibly stain membranes, are impermeable to membranes, and are more fluorescent when bound to membranes than when in solution. Because these dyes stain membranes in an activity-dependent manner, they are ideal for studies of neurotransmitter release mechanisms such as synaptic vesicle recycling, exocytosis, and endocytosis. FM dyes have been used in conjunction with other techniques such as fluorescent calcium indicator dyes and electrophysiological techniques to elucidate mechanisms of presynaptic calcium homeostasis and modulation of neurotransmitter release. Presynaptic membranes have been marked by FM dyes in studies of synaptogenesis and reinnervation. As a probe for endocytosed membranes, these dyes have been used to examine vacuole formation in yeast. These versatile membrane dyes are useful in a variety of applications.