This Special Issue "Clay Mineral Transformations after Bentonite/Clayrocks and Heater/Water Interactions from Lab and Large-Scale Tests" covers a broad range of relevant and interesting topics related to deep geological disposal of nuclear fuels and radioactive waste. Most countries that generate nuclear power have developed radioactive waste management programmes during the last 50 years to emplace long-lived and/or high-level radioactive wastes in a deep underground repository in a suitably chosen host rock formation. For this purpose, most countries have interacted through the International Atomic Energy Authority, which has established safety requirements for the disposal of radioactive waste [[5]] and guidance on how geological disposal facilities should be developed [[6]]. Because radioactive waste retains potentially harmful levels of radioactivity for hundreds of thousands of years, safely disposing of high-level nuclear waste is not a temporary problem, but it involves a million-year solution. [Extracted from the article]