The course is focused on the interaction of photons with organic molecules, molecular crystals and polymers. The primary processes of optical absorption, fluorescence, phosphorescence, non-radiative transitions and generation of excited species, such as excitons, excimers, exciplexes and photoinduced charge transfer states, are discussed in detail. The explanation of basic terms is followed by a complex analysis of photophysical and photochemical processes, such as photochromism, photoconductivity, electroluminescence, photodielectric effect, excitation energy transfer, photosynthesis, photodegradation of materials and some non-linear optical effects. From the application point of view, the principles of basic molecular optoelectronic elements, such as optical memories, photodetectors, solar cells, electroluminescent displays and non-linear optical elements (photorefractive and holographic memories) are discussed. From the chemical point of view, examples are mentioned of relations of molecular structures, physical and chemical properties of materials, types of basic chemical syntheses, and modifications of low-molecular-weight and polymer materials.