  The light-cone approach is reviewed. This method allows to find the
underlying quantum field theory for any integrable lattice model in its gapless
regime. The relativistic spectrum and S-matrix follows straightforwardly in
this way through the Bethe Ansatz. We show here how to derive the infinite
number of local commuting and non-local and non-commuting conserved charges in
integrable QFT, taking the massive Thirring model (sine-Gordon) as an example.
They are generated by quantum monodromy operators and provide a representation
of $q-$deformed affine Lie algebras $U_q({\hat\G})$. Based on lectures
delivered at the $XXX_q$ Karpacz Winter School, Poland, February 14-26, 1994.
