Most of the high-efficiency organic photovoltaic devices (OPVs) reported to date have been fabricated based on the concept of a bulk heterojunction, where a conjugated polymer (donor) and a soluble fullerene (acceptor) form an interpenetrating network featuring a large donor–acceptor interfacial area. In this chapter, we first introduce the fundamentals of OPVs and then review the recent progress related to OPVs based on conjugated polymers. We then discuss the annealing approaches that have been used to optimize the morphologies of the photoactive layers, including thermal annealing and solvent annealing, and describe the engineering of the interfaces at the contacts between the polymer blends and the metal electrodes. Next, we outline the two most common optical methods for improving the light absorption efficiency of OPVs: the use of optical spacers and the triggering of surface plasmons. Finally, we summarize the development of low-band-gap polymers for the absorption of long-wavelength photons from solar irradiation and provide a brief outlook of the future use of OPVs.