The information presented in this review demonstrates the usefulness of the novel GFP marker in environmental microbiology research. Although only available as a marker for a few years, GFP has become another convenient and effective tool in the toolkit of microbiologists studying environmental microorganisms in complex biological systems. The GFP system has numerous advantages over existing marker systems and is particularly useful in visualizing spatial distribution and relationships in situ with existing technology, such as the use of epifluorescent or confocal laser scanning microscopy. The versatility and effectiveness of the GFP in combination with existing microbiological techniques has been demonstrated. GFP has been, and will continue to be, useful in studying various applications of environmental microorganisms from biofilms to bioremediation.