A brief review is given of optical experiments related to the creation and recombination of charged excitons. The fundamental parameters of these states are discussed. This includes their energies, optical transition probabilities, and characteristic times of their formation, thermalisation, and decay. A model of free three-particle charged exciton is shown to describe correctly the results of many experiments. The applicability range of the model is limited by many body effects at higher carrier concentrations and by localisation, typically due to disorder.