This chapter contains sections titled: Introduction Ligand Field Splitting Photophysical Properties Phosphorescent Iridium Complexes Tuning of Phosphorescence Colors in Neutral Iridium Complexes Tuning of Phosphorescence Colors in Cationic Iridium Complexes Tuning of Phosphorescence Colors in Anionic Iridium Complexes Phosphorescent Color Shift in Anionic Iridium Complexes by Tuning of HOMO Levels Controlling Quantum Yields in Iridium Complexes Application of Iridium Complexes in Organic Light-Emitting Devices (OLEDs) Standard OLED Device Architecture Light-Emitting Electrochemical Cell (LEC) Device Architecture