The thesis provides a comparative analysis of good faith in international sales contracts under the United Nations Convention on Contracts for the International Sale of Goods (CISG) and European Union law. While EU law establishes good faith as a mandatory, substantive obligation with specific remedies and comprehensive consumer protections, the CISG, which represents a balance between Civil and Common Law traditions, textually restricts it to an interpretive guideline under Article 7(1). The research defines a serious “dual-regime problem” that creates uncertainty for international traders, as various legal systems enforce distinct norms of behavior and enforcement methods. The study concludes by providing practical compliance strategies – such as particular contract drafting and arbitration selection – to reduce legal risks in cross-border transactions after presenting a trend toward convergence in judicial practice through an analysis of case law, soft law instruments, and doctrinal debates.