Publisher Summary This chapter deals with some current problems of luminescence and energy transfer in organic molecular crystals. The quenching of energy transfer in anthracene crystals at low temperatures is caused by a competition between energy transfer to the guest molecules and to shallow traps that give rise to the emission from impurities or disturbed exciton states at low temperatures. The fluorescence of an anthracene crystal containing small traces of tetracene is measured. The result is that the fluorescence intensity of tetracene is relatively much stronger than the anthracene emission. The tetracene molecules act as traps for the electronic excitation energy, which is traveling in the anthracene crystal. To measure and separate the host and guest quantum yields, one has to measure the total fluorescence spectrum with good resolution. Only in this way is it possible to separate unambiguously the individual contributions from host, guest, and traps to the total emission.