Fluorescent protein-based sensors are essential tools for live cell imaging under fluorescence microscope. They have been evolving from capturing spatio-temporal information of proteins, into visualizing dynamic changes in concentration of signaling molecules and in activation/deactivation states of proteins as changes in fluorescence intensity or fluorescence color during physiological events in living cells. I would like to introduce a unique method I have developed to generate fluorescent protein-based sensors visualizing dynamics of various signaling molecules as fluorescence intensity changes. Moreover, by employing an antibody for the molecular recognition domain, a fluorescent protein-based sensor has innovated into a sensor for detecting a myriad of molecules. These versatile sensors are accomplished by combining molecular evolution and semi-rational molecular design depending on the intracellular physiological phenomena to be revealed.