The dielectric functions and optical constants of various organic solar-cell materials, a total of 20 semiconductors, are summarized. The organic materials described here include CuPc (fluorinated), MEH-PPV, P3HT, PC60BM, PC70BM, PCDTBT, PEDOT:PSS, PMMA, PTB7, and spiro-OMeTAD. For P3HT and PTB7 semiconductors, the optical properties of those polymers blended with PC60BM and PC70BM are shown. For CuPc (fluorinated), MEH-PPV, P3OT, and PEDOT:PSS layers, the anisotropic optical properties are also indicated. Furthermore, it is established that all the dielectric functions described in this chapter can be parameterized by using several transition peaks calculated from the Tauc-Lorentz model, except for PEDOT:PSS, which is modeled further by incorporating the Drude term.