  In this paper we define a quantum version of the ``fusion'' tensor product of
two representations of an affine Kac-Moody algebra.It is replaced by what we
call fusion action of the category of finite-dimensional representations of
quantum affine algebra on its highest weight representations. We construct a
quantum version of the associativity constraint. We give categorical treatment
of the subject and related questions ( like quantum Knizhnik-Zamolodchikov
equations).
