Human development and society progress require solving many pressing issues, including sustainable energy production and environmental conservation. Thermoelectric power generation looks like promising opportunity converting huge heat from the sun and waste heat from industrial sector, housing appliances and infrastructure and automobile and other fuel combustion exhaust directly to electrical energy. Thermoelectric power generation will be of high demand, when technology will be affordable, providing low price, high conversion efficiency, reliability, easy applicability and advanced ecological properties of end products. In this context, organic thermoelectric materials attract great interest caused by non-scarcity of raw materials, non-toxicity, potentially low costs in high-scale production, low thermal conductivity and wide capabilities to control thermoelectric properties. In this chapter, we focus mainly on thermoelectric effect in several organic semiconductors, both crystalline and disordered. We present theory of some transport phenomena determining thermoelectric properties of organic semiconductors, including general expression of thermoelectric effect, percolation theory of Seebeck coefficient, hybrid model of Seebeck coefficient, Monte Carlo simulation and first-principle theory. Finally, a future outlook of this field is briefly discussed.