Osmium(II) complexes possessing beta-diketonate, quinolinate, diimine, and C-linked pyridyl azolate chelates reveal interesting structural and photophysical properties. Spectroscopic and dynamic measurements, in combination with theoretical analyses, have provided an important understanding of the electronically excited state properties of these complexes, such as the energy gap and nature of the lower lying states, rate for intersystem crossing, and the efficiency of corresponding radiative decay and nonradiative deactivation processes. This review also reports on the synthetic processes that lead to the neutral Os(II) and Run complexes that possess two trans-substituted phosphane donor ligands together with two anti-parallel, aligned azolate chromophores. Considerable efforts have been made to focus on utilizing these emitting materials as phosphorescent dopants for practical PLED and OLED fabrication. Consequently, the interplay between these emitting materials and device configurations is discussed. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006).