The decomposition into irreducible modules is determined, for the tens
er product of two arbitrary irreducible integrable highest weight modu
les, for an (untwisted) affine Kac-Moody algebra L. The result is appl
ied to investigate full multiplicity regions and cyclic modules within
the tenser product space for an affine Kac-Moody algebra. The tenser
product decomposition into irreducible modules over L+ V-ir (Coset con
struction), V-ir the Virasoro algebra, is also briefly investigated.