Abstract. Let U , V and W be finite dimensional vector spaces over the same field. The rank of a tensor τ in U ⊗ V ⊗ W is the minimum dimension of a subspace of U ⊗ V ⊗ W containing τ and spanned by fundamental tensors, i.e. tensors of the form u⊗v⊗w for some u in U , v in V and w in W . We prove that if U , V and W have dimension three, then the rank of a tensor in U ⊗ V ⊗ W is at most six, and such a bound cannot be improved in general. Moreover we discuss how the techniques employed in the proof might be extended to prove upper bounds for the rank of a tensor in U ⊗ V ⊗ W when the dimensions of U , V and W are higher.