This chapter provides an overview of modeling organic semiconductors and highlighted the difference between classical materials and organic materials. It discusses microscopic modeling of organic materials from the quantum perspective. The big advantage of using organic semiconductors over inorganic semiconductors is that they can be made using low-energy wet chemistry and the deposition of these materials to form devices can be done at low temperature, saving energy. One key difference between organic and inorganic semiconductors is that the relative permittivity of organic semiconductors is around three while the relative permittivity of inorganic semiconductors such as silicon are around 12. Although organic semiconductors offer many advantages over their traditional inorganic counterparts, they are intrinsically less ordered than crystalline semiconductors. Understanding trap states in organic semiconductors is very important for understanding both transport and recombination. The first attempts at device modeling of organic semiconductors borrowed significantly from inorganic device modeling.