INTRODUCTIONThe rapidly growing arsenal of genetically encoded fluorescent proteins (FPs) obtained from sea creatures has launched and fueled a revolution in live cell imaging. The diverse array of applications benefiting from FPs ranges from markers targeted at organelles and protein fusions designed to monitor intracellular dynamics to reporters of transcriptional regulation and in vivo probes for whole-body imaging and detection of cancer. FPs have enabled the creation of highly specific biosensors to monitor a wide range of intracellular phenomena, including pH and metal-ion concentration, protein kinase activity, apoptosis, membrane voltage, cyclic nucleotide signaling, and tracing neuronal pathways. The purpose of this article is to provide a description of the wide variety of FPs that are currently available.