This perspective illustrates our approach in the design of heteroleptic cationic iridium(III) complexes for optoelectronic applications, especially as emitters in electroluminescent devices. We discuss changes in the photophysical properties of the complexes as a consequence of modification of the electronics of either the cyclometalating (C^N) or the ancillary (N^N) ligands. We then broach the impact on these properties as a function of modification of the structure of both types of ligands. We explain trends in the optoelectronic behaviour of the complexes using a combination of rationally designed structure-property relationship studies and theoretical modelling that serves to inform subsequent ligand design. However, we have found cases where the design paradigms do not always hold true. Nevertheless, all these studies contribute to the lessons we have learned in the design of heteroleptic cationic phosphorescent iridium(III) complexes.