A compendium representing the current state of the art in the modelling, simulation and physics of the interaction of hydrogen and helium with plasma facing materials in fusion reactors. This is the topic that will determine the success of the production of energy by future Tokamak reactors and it is here discussed by the world's experts. Topics covered are recycling of hydrogen isotopes; wall fuelling and wall pumping; active control of hydrogen recycling; hydrogen and helium behaviour in solids and liquid metals; and databases for recycling.