This book describes how the international sales of goods have generally been ruled by either English Law or Civil Law, which has often posed problems due to different approaches regarding certain principles and institutions. It clarifies how the Vienna Convention on Contracts for the International Sale of Goods of 11th April, 1980, tried to harmonise these differences with a codification technique, typical of civil law, giving privilege to rules of civil law most of the time, but also introducing institutions from common law, that are not incompatible with civil law. It explains why the general principles of civil law and of UNIDROIT help with this goal of harmonisation, integrating the loopholes of the UN Convention on Contracts for the International Sale of Goods (CISG) during its interpretation. The work demonstrates why codification prevails over common law in the CISG most of the time, giving certitude and sophistication to this matter, which is vital for global commerce.