Fluorescent proteins are generating fresh insight into plant cell function by providing new opportunities to visualize structure and dynamic events in live cells. Novel and transient structures, such as discreet locations in the nucleus where activated photoreceptor proteins accumulate, have recently been identified with fluorescent protein tags. Fluorescent proteins have also enabled the discovery of new dynamic molecular behaviors, such as the repositioning of cortical microtubules by polymer treadmilling. The early potential of fluorescent proteins to reveal protein interactions in living cells is being realized, as demonstrated in recent studies of transcription factors and signal transduction proteins. A promising new approach to the creation of fluorescent-protein-based biosensors has produced an exciting family of tools for visualizing small sugars, and perhaps will produce a wide variety of other small molecules in the future.