Transition metal complexes have attracted interest in the fields of cancer therapy, medical and environmental detection, and materials of organic light-emitting diodes (OLED). Recently, iridium(iii) complexes have been explored in a reaction based way for luminescence monitoring and imaging due to their salient photophysical properties, including large Stokes shifts, long-lived luminescence and high luminescent quantum yields. This frontier article will introduce recent developments and applications of iridium(iii) complexes as luminescent probes for ions and biomolecules. Our outlook for this class of complexes is also discussed.