The development of fluorescent proteins as molecular tags over the past decade has spurred a revolution by allowing complex biochemical processes to be correlated with the functioning of proteins in living cells.   Fluorescent proteins such as green fluorescent protein (GFP) from the jellyfish Aequorea victoria and its variants can be fused to virtually any protein of interest to analyze protein geography, movement and chemistry in living cells.   As such, they have provided an important new tool for understanding protein function, filling an urgent need now that the genome sequence of many organisms is complete.  The modified GFPs have been used as markers to track and quantify individual or multiple protein species, as probes to monitor protein-protein interactions, and as photochemically modulatable proteins to highlight and follow the fate of specific protein populations within a cell.  The focus of this talk will be on the kinetic microscopy methods of photobleaching and photoactivation that are being used to monitor the appearance, location, movement and degradation of GFP fusion proteins in living cells.   Results from these applications are providing profound new insights into protein function and cellular processes in the complex environment of the cell.