In-situ hydrogen production from natural gas reservoirs (IHP-NG) is an emerging pathway for low-carbon hydrogen by converting methane-rich subsurface resources while retaining carbon-bearing byproducts underground. This review critically evaluates the reaction mechanisms, experimental and modelling evidence, techno-economic position, life-cycle implications, and deployment challenges of IHP-NG. Unlike heavy-oil in-situ combustion gasification, IHP-NG is governed mainly by methane conversion reactions, including steam methane reforming, partial oxidation, autothermal reforming, methane cracking, dry reforming, and water--gas shift, alongside competing pathways such as methanation, reverse wat... More >