Recent improvements in the power conversion efficiencies of organic solar cells have been derived through a combination of new materials, processing, and device designs. A key factor has also been quantum-chemical studies that have led to a better understanding not only of the intrinsic electronic and optical properties of the materials but also of the physical processes that take place during the photovoltaic effect. In this chapter we review some recent quantum-chemical investigations of donor-acceptor copolymers, systems that have found wide use as the primary absorbing and hole-transport materials in bulk-heterojunction solar cells. We underline a number of current limitations with regard to available electronic structure methods and in terms of the understanding of the processes involved in solar cell operation. We conclude with a brief outlook that discusses the need to develop multiscale simulation methods that combine quantum-chemical techniques with large-scale classically-based simulations to provide a more complete picture.