The procedure used in quantum field theory and the many-body problem in quantum mechanics in which theories involving fermions have the fermions replaced by an effective field theory with bosons. In one-space-dimensional systems the transformation from fermion fields to boson fields is exact. For higher-dimensional systems, bosonization is a procedure that, in general, can only be carried out approximately; it is, for example, only valid as a low-energy approximation. The derivation of an effective field theory for mesons, starting from quantum chromodynamics, is an example of an approximate bosonization applicable to low energies. The transformation to the description of an electron gas in terms of plasmon variables is another example of approximate bosonization.