Overview: The main rationale for declining long‑term discount rates results from uncertainty about the future. This uncertainty can be shown to cause declining discount rates over time; the discount rate is estimated for up to 300+ years and it is dropping from 3.5% pa to 1% pa (in 300 years). Discount factors are the factors by which future cash flows must be multiplied to obtain the net present value (NPV), i.e., if the discount rate is d, the discount factor for year i is represented by 1/(1+d)i . Professor Mark Freeman’s work has contributed to debates and policy development in several contexts, including the OECD’s International Transport Forum, HM Treasury’s ‘Green Book’ which guides financial decision making, and the Netherlands Ministry of Finance. Professor Freeman is also turning his attention to long‑term climate change and counter‑terrorism and has held initial discussions with relevant agencies.