Until recently, cytoskeleton research has relied primarily on immunofluorescence microscopy techniques, requiring fixation and hence killing of the specimen before the analysis. The sole method for visualizing cytoskeletal dynamics in living cells has been the microinjection of purified and fluorescently labelled protein, but technical difficulties have precluded its widespread use. The recent introduction of green fluorescent protein (GFP) has enabled visualization of proteins and cytoskeletal dynamics with only minimal perturbations of the living cell and has opened new horizons for studying the cytoskeleton.