Genetically encoded calcium indicators (GECIs) are powerful tools to monitor the dynamics of calcium ion (Ca(2+)) in living cells and organisms. With the help of GFP technology and DNA engineering, a dozen sets of GECIs have been developed so far. Their application has been widely extended into the analysis at the subcellular local, single and population of cell. In the past decades, GECIs have been dramatically improved in their performance and are becoming more and more useful for live imaging. In this review, the progress in the development of GECIs is discussed by introducing the history and emerging GECIs, which would help the selection of the appropriate GECI for a given application.