Optical spectra associated with transitions that create or annihilate charged excitons X -or X+ can be observed in quantum well heterostructures containing an electron gas or a hole gas, respectively. A review is given of properties of trion states in CdTe quantum wells in zero field and of the magnetic field-dependence of the circular polarization and oscillator strength of the trion optical resonance. The possibility that disorder is needed to stabilise trion states in concentrated 2D electron or hole systems is discussed.PACS numbers: 73.20. Dx, 78.66.Ηf, 71.35.Ji, 78.20.Ls In 1958, M.A. Lampert [1] extended the analogy between the exciton Χ in a semiconductor and the hydrogen atom H in free space by postulating the existence of two types of charged exciton species or "trions" . The first is X -(two electrons bound to one hole). For mh » me , this is a close analogue of the H -ion where two electrons with antiparallel spins move in a very diffuse orbital around a proton. The second species is X+ (two holes and one electron). For mh » me , this is analogous to the Η2 molecular ion where one electron moves in a molecular orbital around two protons (Lampert suggested the nomenclature X2 , but "X+" has become more usual).Both kinds of trion have only a very small binding energy and their observation in bulk semiconductors has proved very elusive. However, the calculated binding energy of trions is enhanced by an order of magnitude in 2 dimensions [2]. Moreover, an excess of electrons (or holes) can readily be produced in a quantum well heterostructure by modulation doping, favoring the formation of X - (or X+) in the well.The first clear identification of the trion X -was made in 1993, in the optical spectra of modulation-doped CdTe quantum wells containing 10 10 to 10 11 electrons/cm2 [3]. Trions X- [4] and X+ [5] were then identified in GaAs (99)