Survey.- to exciton spectroscopy.- Properties of biexcitons.- Biexcitons-bose condensation and optical response.- Properties of electron-hole drops in germanium crystals.- Theory of electron-hole drops in germanium and silicon.- Spectroscopic study of exciton-exciton interaction (Biexcitons, drops) in semiconducting crystals.- Exciton condensation in germanium.- Gigantic oscillator strengths inherent in exciton complexes.- Interaction between excitons at high concentration.- Theory of stimulated emission by excitons.- Experimental investigation of the competition of stimulated emissions involving exciton.- Experimental studies of excitons at high densities.- Tests of validity of spatial dispersion theories on lead iodide crystal spectra.- Medium and high polariton densities.- Polaritons at high light intensities and in bose condensed exciton systems.