Fluorescent molecules serve as valuable tools for the detection of numerous biochemical phenomena and have been employed for protein localization, quantitation of gene expression, detection of nucleic acids, cell sorting and determination of chemical concentrations. However, the use of such techniques generally requires significant nonphysiological perturbations to the biological system being studied; therefore, they are not always appropriate for the observation of dynamic phenomena. Green fluorescent protein (GFP), cloned from jellyfish, has been used to overcome many of these problems. It is a small, extremely stable fluorescent protein that has been successfully expressed and detected in a wide variety of organisms, both in intact form and fused to other proteins. This overview unit describes the use of this proteinaceous fluorophore for in vivo observation of cellular phenomena.