The states of charged magnetoexcitons in two-dimensional systems are consid
ered. Exact optical selection rules for intra- and inter-band processes are
discussed. The effect of excess electrons on internal transitions of negat
ively charged excitons X in quantum wells is studied experimentally and the
oretically. An experimentally observed blue-shift with excess electron dens
ity is explained in terms of collective excitations, magnetoplasmons bound
to a valence band hole.