Algorithms for resolution of singularities in characteristic zero are based on Hironaka’s idea of reducing the problem to a simpler question of desingularization of an “idealistic exponent” (or “marked ideal”). How can we determine whether two marked ideals are equisingular in the sense that they can be resolved by the same blowing-up sequences? We show there is a desingularization functor deﬁned on the category of equivalence classes of marked ideals and smooth morphisms, where marked ideals are “equivalent” if they have the same sequences of “test transformations”. Functoriality in this sense realizes Hironaka’s idealistic exponent philosophy. We use it to show that the recent algorithms for desingularization of marked ideals of Włodarczyk and of Kollár coincide with our own, and we discuss open problems.