24In that respect, it should be borne in mind that the Commission may adopt guidelines on the exercise of its powers of assessment, particularly in State aid matters. In so far as those guidelines do not contradict Treaty rules, the policy rules which they contain are to be followed by that institution (see Case 310/85Deufil v Commission [1987] ECR 901, paragraph 22; Case C-313/90CIRFS and Others v Commission [1993] ECR I-1125, paragraphs 34 and 36, and Case C-311/94IJssel-Vliet [1996] ECR I-5023, paragraph 42).