This research, entitled “The Annulment of Defective Administrative Decisions,” provides an analytical study of the principle of annulment of administrative acts, whether individual decisions or regulatory decrees, within the framework of the principle of legality, which constitutes the foundation of administrative action. The study explains that the administration may, whenever it discovers that its decision violates the law, revoke or amend it, either upon the request of an interested party or on its own initiative, in order to rectify any action that contravenes legal rules. The annulment of a defective administrative decision requires that the defect be subsequent to the issuance of the decision. However, if the defect is contemporaneous with its issuance, the administration may withdraw the decision within the legally prescribed period for appeal, and in such a case, the effect of annulment is limited to the future only, without retroactive impact. As a general rule, the administrative authority that issued the decision is the one competent to annul it, provided that the annulment is carried out under the same conditions and procedures as those required for its issuance. An administrative decision is deemed valid when it is issued by a competent authority, based on legally established facts, and aimed at achieving the public interest. However, when the administration deviates from the principle of legality, judicial oversight becomes necessary. An interested party may file a lawsuit for annulment before the administrative judiciary to challenge the unlawful decision. The administrative judge is authorized to examine the legality of the contested decision, and if it is found to be contrary to the law, the judge annuls it—without possessing the authority to modify or replace the decision. Furthermore, judicial annulment judgments have absolute authority, meaning their effects extend to all persons, whether or not they were parties to the case. Consequently, the administration is obliged to implement the judicial decision of annulment by issuing a formal act enforcing the judgment, thereby restoring the situation to its prior state before the annulled decision was issued and taking all necessary measures to ensure full execution of the ruling and its legal consequences. Keywords: Administrative annulment, Methods and rules of annulment, Competent authority for annulment, Judicial annulment, Content of judicial annulment, Its authority، Its effects.