Green fluorescent protein (GFP) Firstly discovered from the jellyfish Aequorea victoria and other bioluminous marine animals are widely used in biological science as a universal fluorescent tag. Many scientists have focused their efforts on identification and development of fluorescent proteins with novel characteristics and enhanced properties, to be used for visualization of structural organization and dynamic processes in living cells. The discovery of green fluorescent protein (GFP) followed by purification and elucidation of its molecular structure introduced a new and promising marker for biological studies. GFP is intrinsically fluorescent, has a low toxicity, pH and heat stability and easily imaged and quantified. This review article focuses on the discovery, diversity of fluorescent patterns and colors, other fluorescent protein, purification, structure, properties, and numerous applications of GFP in the biological research such as in vivo imaging of cancer, in promoter and protein fusion constructs, biosensors, Ex vivo imaging, Imaging the efficacy of therapeutics.