This study comparatively analyzes the Reasonable Expectation of Success (RES) requirement in the assessment of inventive step under the jurisprudence of the European Patent Office (EPO), the Unified Patent Court (UPC), and the United States and examines its implications for Korean patent law. In decisions T 0136/24 and T 0122/23, the EPO Boards of Appeal rejected the proposition that the initiation of a clinical trial or the disclosure of a clinical trial protocol automatically gives rise to a reasonable expectation of success; importantly, however, in T 0122/23, the Board ultimately found that a reasonable expectation of success existed on the basis of additional prior art evidence. In Amgen v. Sanofi (UPC_CoA_528/2024), the UPC Court of Appeal articulated guiding principles for assessing inventive step, including the role of a reasonable expectation of success in that assessment. This study maps technology-specific RES thresholds across predictable arts, pharmaceutical and biotechnology inventions, polymorph inventions, and AI-related inventions. It further argues that the Korean Supreme Court doctrines of “difficulty of composition” (2019Hu10609, Apixaban) and “ease of prediction” (2016Hu502) are functionally comparable to RES but differ in the degree to which the relevant assessment factors are explicitly articulated and structured. Rather than formally adopting RES as an independent statutory requirement for inventive step, this study recommends refining existing Korean doctrines through clearer assessment factors, technology-sensitive standards, and more precise evaluation of prior art relating to clinical trial protocols.