EN
A brief review is given of optical experiments related to creation and recombination of charged excitons. Their results are analyzed in terms of a simple three-particle free trion model. It is shown that the simple model explains well such results as temperature variation of lifetime and absorption intensity or photoluminescence line shape and a decrease in absorption oscillator strength per carrier with increasing carrier concentration. The range of applicability of the simple model is examined and found to be limited by two factors: disorder-induced localization and many-body interactions occurring and high carrier sheet concentrations (of order of 10^{11} cm^{-2}).