There is much current interest in the use of organic materials for applications in the opto-electronics industry. This thesis is concerned with the investiga-tion into t he t heoretical properties of so called Organic Semiconductor Lasers (OSLs), which may have applications in display technologies and low-power communications as a low cost alternative to inorganic devices. The work presented in this thesis represents, to the best knowledge of the author, the only theoretical investigation of OSL's to date. Many of the physical processes associated with the electrical excitation of OSLs are yet to be understood completely, the analyses in this thesis are therefore mainly concerned with the optical properties and lasing thresholds of the OSL structures, demonstrating how the differing optical properties of organic compared to in-organic materials effect the performance of various device con-figurations. This analysis is extended to theoretical devices such as Circular Grating CG-DFB OSLs, which are yet to be demonstrated experimentally. Due to well published and thorough experimental investigation of a certain group of materials, the Alq3 : DCM combination, it has recently become possible to begin to investigate the lasing thresholds of these lasers in terms of the amount of current they would require under electrical pumping schemes. Initial data showing how the threshold current is affected by changes in the geometry of a range of devices is also presented.