This commentary focuses on Article 3.3.1 of the UNIDROIT Principles of International Commercial Contracts (PICC) concerning contracts that infringe mandatory rules. Art 3.3.1 governs a specific aspect of the mandatory rules regime, namely the effect that their infringement produces on a contract. The relevant mandatory rules, however, are not set forth in the PICC; rather, the PICC defer to the binding norms from which such rules originate. Thus, the definition of what constitutes illegal conduct is to be found in those binding norms. Similarly, Art 3.3.1 defers to Art 1.4 PICC with respect to the applicability of any mandatory rule, which, in turn, defers to the relevant rules of private international law. This commentary discusses the scope of application of Art 3.3.1, with particular emphasis on the effects of mandatory rules whether or not expressly prescribed, effects upon the contract and remedies under the contract, and the importance of mandatory rules.