This chapter discusses the processes of stimulated emission and optical gain in organic compounds given, followed by a detailed description of the experimental technique broadly used in order to check the presence of optical gain in organic materials and to quantitatively measure itsvalue. The emission spectra of conjugated polymers are, in general, red shifted compared to the absorption spectra; thus, the auto-absorption is strongly reduced. A single molecule possesses energetic states corresponding to electronic, vibronic, rotational, and translational quantum states. Excitons are the basis for the description of primary elementary excitation in semiconductor crystals. An exciton can be defined as an elementary excitation constituted by a hole into the ground state and an electron into an excited state, bound together by Coulomb interaction. Electronic transitions in organic solid compounds usually occur with a variation of both the electronic and the vibrational state.