Organic molecules are currently being investigated with regard to their application as active components in semiconductor devices. Whereas devices containing organic molecules for the generation of light - organic light emitting diodes (OLED) - have already reached the market (they e.g. display information on mobile phones), transistors where organic molecules are used to actively control currents and voltages are still in the development stage. In this book the principle problems related to using organic materials as semiconductors and to construct functioning devices will be addressed. A particular emphasis will be put on the difference between inorganic semiconductors such as Si, Ge and GaAs and organic semiconductors (OSC). The special properties of such soft matter require particular approaches for processing characterization and device implementation, which are quite different from the approach used for conventional semiconductors.