Abstract. We introduce the notion of double cosets relative to two fusion subcategories of a fusion category. Given a tensor functor F : C → D between fusion categories, we introduce an equivalence relation ≈ F on the set Λ C of isomorphism classes of simple objects of C, and when F is dominant, an equivalence relation ≈ F on Λ D . We show that the equivalent classes of ≈ F are cosets. We also give a description of the image of F when it is a normal tensor functor, and we show that F is normal if and only if the images of ≈ F equivalent elements of Λ C are colinear. We study the situation where the composition of two tensor functors F = F ′ F ′′ is normal, and we give a criterion of normality for F ′′ , with an application to equivariantizations. Lastly, we introduce the radical of a fusion subcategory and compare it to its commutator in the case of a normal subcategory. We also give a description for the image of a normal tensor functor between any two fusion categories.