The advantage of using phosphorescent transition metal-ligand complexes in optoelectronic applications such as organic light-emitting diodes (OLEDs) and light-emitting electrochemical cells (LECs) are described and evaluated. Additionally, different device constructions utilizing phosphorescent transition-metal complexes like iridium(III) mixed-ligand complexes and ruthenium(II) systems are reviewed and specified. Diverse host materials in which the phosphorescent emitters can be placed are discussed, such as small organic molecules and a few polymeric systems, and alternative processing technologies are briefly compared. Recent developments in the synthesis of iridium(III) triplet emitters are discussed. Different device architectures require different kinds of metal-ligand complexes. The different synthetic routes leading to charged and non-charged complexes are briefly discussed.