Over recent years several new classes of conjugated polymer and molecular semiconductors have shown promise as high mobility materials for organic field-effect transistors. We have been interested in understanding their fundamental charge transport physics and the underpinning relationship between molecular structure and charge transport properties. Information about their electronic structure can be gained from measurements of fundamental transport coefficients such as Hall effect and Seebeck coefficient. In particular, the Seebeck coefficient provides direct insight into the density of states available for transport and complements measurements of the carrier mobility as a function of temperature and carrier concentration. In this presentation we will review our current understanding of the charge transport and thermoelectric properties of these materials and discuss these in the context of applications in electronics, optoelectronics and thermoelectrics.