This paper presents Nagra’s systematic methodology for developing safety scenarios for the post-closure safety case in support of the Swiss general licence application for deep geological disposal of radioactive waste. The approach builds on the assessment basis, performance assessment outcomes, and international guidelines. It combines top-down safety-function reasoning with bottom-up FEP screening to ensure comprehensive and traceable coverage of system evolutions. Four safety scenario categories are defined: the reference safety scenario describing expected evolution, alternative safety scenarios addressing credible deviations, hypothetical “what-if?” cases testing system robustness, and stylised safety scenarios of future human actions. Each scenario is complemented by variants and calculation cases designed to capture conceptual and parametric uncertainties relevant for post-closure safety. The resulting scenario set provides a structured and manageable input for radiological consequence analyses and confidently bounds the radiological consequences of a comprehensive range of plausible and hypothetical evolutions. Through conservative assumptions, bounding variants, and structured safety scenario categorisation, the methodology robustly supports the demonstration of post-closure safety and ensures regulatory compliance. The work establishes a defensible and scientifically grounded safety scenario framework that underpins the post-closure safety assessment for deep geological disposal in Switzerland.