In many scenes in current biology, fluorescent proteins have been utilized to visualize target proteins and structures in live cells and tissues. The usage of fluorescent proteins is not limited to labeling, but they can also be used as reporters of intracellular environments, such as ions and adenosine triphosphate concentrations, pH, and temperature. In combination with FRET or FRAP techniques, fluorescent proteins even allow us to trace the molecular‐level dynamics of protein structures in live cells. The switchable capability of fluorescence emission has allowed us to realize superresolution microscopy techniques, research that was awarded the 2014 Nobel Prize in Chemistry. Recently, chemiluminescence, which was Professor Shimomura’s main research interest, became available to image intracellular targets even without light illumination. The discovery of fluorescent protein thus initiated the idea of using light‐protein interactions for biological research. For example, optogenetics can manipulate cell and animal activities by light‐induced conformational change of rhodopsin.