I shall explore in this article the hypothesis that structures are relations between the components of complex entities. After having introduced hylomorphism, its major advantages and the major views of the nature of structures, I shall introduce the distinctions between external and internal relations and the one between symmetrical and non-symmetrical relations. I shall also describe the theory of non-symmetrical relations that I accept, i.e., the O-Roles theory, as most structures seem to be external and non-symmetrical relations. Later on, I shall develop my hypothesis and I shall deal with one objection, i.e., that relations necessarily come together with Bradley’s vicious regress. Finally, I shall also hypothesize that complex entities are different from structures and from their structured components. In this respect, I shall defend a “simple view” of complex entities. Dwelling on this view, I shall also reply to some recent objections against hylomorphism.