. Research into phosphorescent Ir(III) complexes has grown immensely since their first report. Talented chemists have successfully synthesized complexes capable of emitting from the UV to the near-IR regions of the electromagnetic spectrum. Tuning the emission energy in a selective manner requires knowledge of how ligand substitution affects not only energy levels such as the HOMO and LUMO, but also the emissive triplet energy level. This review describes fundamental principles involved in energy-level engineering and substituent selection as well as fundamental methods of device c...