Organic electronics emerged as a promising research field many decades ago and has matured in recent years to a technology that is already present in various electronic devices. However, a full understanding of the involved fundamental charge-transport mechanisms is still lacking, which inhibits the establishment of an in silico materials design encompassing molecular properties, supramolecular morphology, and charge transport characteristics. Different theoretical approaches try to remove this roadblock for a more rapid development of this field based on deeper theoretical insight and rational materials design. In this chapter, starting from the Kubo formalism, we discuss several theoretical approaches to describe charge-transport characteristics of organic semiconductors that can be applied to a broad spectrum of potential candidate materials. We discuss how these approaches can be connected to specific materials by means of ab initio simulations and present illustrative examples for prototypical molecular systems.