  This contribution reviews recent progress in constructing affine Lie algebras
at arbitrary level in terms of vertex operators. The string model describes a
completely compactified subcritical chiral bosonic string whose momentum
lattice is taken to be the (Lorentzian) affine weight lattice. The main feature
of the new realization is the replacement of the ordinary string oscillators by
physical DDF operators, whereas the unphysical position operators are
substituted by certain linear combinations of the Lorentz generators. As a side
result we obtain simple expressions for the affine Weyl translations as Lorentz
boosts. Various applications of the construction are discussed.
