We define a family of universal finite-dimensional highest weight modules for
affine Lie algebras, we call these Weyl modules. We conjecture that these are
the classical limits of the irreducible finite--dimensional representations of
the quantum affine algebras. We prove this conjecture in the case of affine
sl_2. We establish a criterion for these modules to be irreducible and prove a
factorization theorem for them in the general case