This commentary analyses Article 7.1.1 of the UNIDROIT Principles of International Commercial Contracts (PICC) concerning the definition of non-performance. Article 7.1.1 defines non-performance as ‘failure by a party to perform any of its obligations under the contract, including defective performance or late performance’. Non-performance under Art 7.1.1 also includes violations of accessory obligations, such as a duty of confidentiality or the duty to co-operate, including the accessory obligations of the aggrieved party. This commentary discusses reasons for non-performance, the concept of ‘default’, the relationship between non-performance and non-conformity, vicarious liability for contractual obligations, and the burden of proof relating to non-performance.