Are molecular semiconductors a reality? The previous chapters have been partly devoted to answering this question. It has been seen that beside the semantic problem of their definition, the denomination “molecular semiconductor” raises several fundamental points. A molecular material is considered to be a semiconductor if it possesses a reasonable mobility of charge carriers and if it demonstrates the existence of a band gap of the order of 1–2 eV. The molecular material must also be able to form typical semiconductor-based devices: junctions and solar cells. With these criteria in mind, no example of molecular semiconductor has been found, up to the present time, throughout the literature. Polymeric systems may potentially lead to high-mobility charge carriers, but either they cannot be doped and are insulators, or they are too impure and too inhomogeneous to exhibit experimentally accessible high mobilities. The purity of molecular crystals is satisfactory but the corresponding charge carrier mobilities are limited by the poor intersubunit overlap. Charge transfer complexes and radical ion salts presumably form semiconducting materials but at very low temperatures (T < 60 K) and with very small band gaps (Eg ~ 0.1 eV).