The book provides a comprehensive presentation of the law of supplementary performance under the UN Sales Convention and offers a detailed overview of the prevailing opinions in the case law and literature of many member states of the CISG. A particular focus is placed on the comparison with the supplementary performance concept of the German Civil Code. The author addresses various contentious issues, such as the modalities of supplementary performance, the right of choice, legal consequences, and the impact of the UN Limitation Convention, as well as the special questions arising from the conceptual differences between Civil and Common Law, particularly the issues of actual performance and specific performance. It thoroughly examines Article 28 of the CISG as a potential limitation on the right to supplementary performance and the question of the buyer's self-execution of defect rectification under the CISG.