First discovered at the beginning of the 20th century but still only partially understood today, organic semiconductors combine the electrical and optical properties typical of inorganic semiconductors with properties such as flexibility, low cost, and structural tunability via chemical modification. They are of significant interest due to their potential for optoelectronic applications such as displays, photosensors and solar cells. Crystalline organic charge-transfer compounds, combinations of two or more organic molecules in which one species acts as a donor of electric charge and the other as an acceptor, could provide new properties or improved performance to increase the range of application of organic semiconductors. Because of the hierarchy of bonding in these molecular crystals, the subtle interplay of electronic and vibrational states has far more influence on their properties than on those of covalent inorganic crystals. The further development of many applications of such compounds is limited by the lack of understanding of exciton dissociation and charge recombination processes and how these processes depend on the electronic and electron-vibration interactions. The charge-transfer states formed at the donor-acceptor interface play a key role, and both experimental and theoretical analyses depend on the arrangement of the donor and acceptor molecules at the nanoscale. By combining optical and transport measurements such as resonant Raman scattering, transient absorption and photocurrent with quantumchemical calculations it is possible to advance our understanding of the physics of these complex materials, paving the way for their application in 21st-century opto-electronic devices.