The charge transport in organic semiconductors and the contact characteristics in organic devices have been investigated. A universal mobility model which can be applied to organic semiconductors has been derived. This model is based on the variable range hopping theory and can explain the temperature, electric field, and carrier concentration dependencies of mobility. An electric field dependent transport energy model has been formulated. This model extends Arkhipov's transport energy theory by considering the effect from an electric field on hopping transport. It has been shown that the transport energy increases with increasing temperature, but the effect from an electric field on transport energy is not monotonic. This information can be used to analyze the mobility, which increases with the electric field at a low electric regime, while it decreases at a high electric field regime. A diffusion-controlled injection model has also been obtained. This model is based on drift-diffusion theory and multiple-trapping transport theory. This model can explain the injection current characteristics of temperature, electric field, and the energy barrier between metal and organic semiconductors. Good agreement between model and experimental data has been found. The master equation has been proved to be a good choice in describing the hopping transport in organic semiconductors. An injection model based on the master equation has been derived. This model considers the effect of image charge force and back-flow hopping on the net injection current. It concludes that the Richandson-Schottky model (RS) is valid only in a low electric field and low energy barrier between metal and organic semiconductors. At the same time, a model describing space-charge-limited current (SCLC) has been built. This model is based on hopping transport and density of states (DOS) with a Gaussian function. By treating the states at the center of a Gaussian DOS as transport sites and those at the tail of a Gaussian DOS as trapping sites, we conclude that SCLC controlled by a Gaussian DOS distribution obeys the relation formulated by Mott and Gurney only in the low current regime and that the field-dependent mobility changes this relationship slightly.