A narrative of the initial state and expected evolution of the repository system is presented in the following sections. It is based on sound scientific understanding and accounts for uncertainty, as documented in the assessment basis reports (Fig. 1‑1), and as brought together in more detail in NAB 24‑20 Rev. 1 (Nagra 2024m). Arguments and evidence supporting key claims within this account of the expected evolution are validated by the performance assessment (PA), as summarised in Chapter 6.