Abstract: The past decade has seen rapid growth in the study of the optoelectronic properties of organic semiconductors and their application to practical devices. Given that organic solids are typically held together by weak van der Waals bonds, organic semiconductors do not require substrate lattice matching. This relaxed condition permits organic optoelectronic devices to be deposited on a variety of substrates, allowing for ease of fabrication and new device architectures that can be both transparent and flexible structures. Our work has focused on: (i) efficient and spectrally pure blue phosphorescent organic light emitting devices (OLEDs) for high performance display and solid-state lighting applications, and (ii) the study of the non-radiative loss mechanisms that limit OLED efficiencies and the realization of electrically pumped organic lasers. As an alternative to coherent emission from organic materials, we explore the behavior of Frenkel excitons in resonant optical microcavities and specifically, the strongly coupled exciton-photon state. For blue phosphorescent OLEDs, our focus was on the characterization of new materials and device architectures to improve OLED efficiencies. We developed phosphorescent materials that tend to trap charge such that excitons are formed directly on the phosphor guest as opposed to the host. Given the inherent challenges associated with host-guest energy transfer in the deep blue, exciton formation by direct charge-trapping offers an efficient route to efficient and saturated blue electrophosphorescence. Our analysis of electrically pumped organic lasers suggests that lasing by direct carrier injection into an OLED-type structure is complicated by intense exciton quenching at large drive current densities. Hence, we studied strong exciton-photon coupling in small molecular weight amorphous and polycrystalline organic as well as in hybrid organic-inorganic structures. We explore the basic properties of the coupled state in organic materials characterized by strong vibronic character and large rates of singlet-triplet intersystem crossing. Through these experiments, a better understanding of strong coupling is obtained, on the path to developing a strongly coupled coherent light emitter, or 'polariton laser.'