This chapter explores the several processes affecting the amplified spontaneous emission (ASE) properties of organic active waveguides, beyond their dependence on the gain cross section of the active material. It begins from the fundamental aspects of the waveguide operation, addressing the importance of the dielectric properties of the active layer and of its thickness on the waveguide modes number and structure. The chapter provides several examples of processes able to modify the ASE properties of a given active molecule. It discusses the role of the waveguide thickness, for molecules dispersed in an inert matrix or in a neat film. The chapter describes the main features of light waveguiding, starting from a simple ray optic description, and then moving to an electromagnetic wave description based on the Maxwell equations. The characterization of the ASE properties of an organic thin film exploits light waveguiding in the active film and it is experimentally relatively easy.