Organic semiconductors and devices represent a new field for physics, material sciences, engineering and chemistry. They lead to a variety of new applications such as organic transistors for flexible and printable integrated circuits and organic light emitting diodes (OLEDs), which are already used as a new technology for a variety of flat panel display applications. Alq3 (tris(8-hydroxyquinoline)aluminum(III)) is one of the most frequently used materials in this field and therefore was chosen as a model material. This book demonstrates how the combination of a large range of scientific methods leads to conclusive understanding of the properties of such organic materials. Low-temperature and site-selective photoluminescence emission and excitation spectroscopy, differential scanning calorimetry and X-ray analysis of powder samples with synchrotron radiation and vibrational analysis (IR- and Raman spectroscopy) are used to determine the molecular properties. Delayed fluorescence and transient measurements of both the photoluminescence and the delayed luminescence are introduced and used to investigate the characteristic properties and dynamics of the electronic excited singlet and triplet states, which are of high importance in devices such as OLEDs.