This lecture covers the interaction of electromagnetic radiation with organic molecules, leading to the absorption of visible light and the generation of color in organic dyes. It explains the electronic excitation of molecules by absorption, the fluorescence phenomenon, and the stimuli-responsive changes in the electronic system. The lecture also delves into the properties of organic electronic materials, the role of extended conjugated systems, and the electron density distribution in conjugated systems. Furthermore, it discusses the types of pi-pi interactions, the single-crystal structure of pentacene, and the bands in crystals of conjugated molecules. The charge transport in organic thin film devices and possible applications of organic field-effect transistors are also explored.