Organic semiconductors in general are a family of electronic materials that are basedon π-conjugated carbon atoms. In the last three decades electronic devices based onthis family of materials, such as field-effect transistors and light emitting diodes, haveattracted much attention as possible inexpensive and flexible alternatives to inorganicdevices. Although we witnessed considerable progress in the introduction of new com-mercial applications that are based on these materials, the nature of charge transport inthese organic materials and devices has not been understood very well. The main goalof this thesis is to theoretically investigate the charge transport properties of organicsemiconductor materials and devices.Charge transport properties presented here are investigated in the framework of vari-able range hopping theory. In a previously published paper by Vissenberg, a percolationmodel has been developed in order to explain the temperature dependence of hoppingmobility in organic semiconductors. One of our main theoretical goals is to develop dif-ferent models that can explain the dependence of the mobility in organic semiconductorson electric field, temperature, carrier concentration, and doping and trap concentration.A both temperature and electric field dependent mobility model is developed based ona modified Miller-Abrahams rate equation. The carrier concentration dependent mo-bility is formulated assuming a Gaussian density of states. A unified mobility modelis presented which can explain the temperature, electric field and carrier concentrationdependence. The doping and trap dependent mobility model is obtained by assuming asuperposition of two exponential density of states functions.The charge injection process between metal and organic semiconductor is examined fororganic light-emitting diodes. For this goal we develop both a diffusion-controlled and amaster equation based injection model. These two models can explain the dependenceof the injection current on the temperature, electric field and barrier height. Goodagreement between calculation and experimental data is found. We examine closely the space charge limited current (SCLC) and the effect of the Fermi-Dirac statistics on the transport energy. It is found that the SCLC due to a Gaussiandensity of states is similar to SCLC controlled by shallow traps in regular semiconductors.The Fermi-Dirac statistics plays an important role for transport energy, even at low temperature.Finally, analytical models applicable to organic thin film transistors and to unbipolarorganic light-emitting diodes are presented.