The thermally activated processes of hydrogen diffusion in wall materials such as stainless steels and inconels and of its surface release (including desorption) play an essential role in the hydrogen recycling in many fusion devices. Non thermally activated processes such as deposition, implantation and direct reflection (backscattering) of atomic particles, particle- and photon-induced desorption and trapping effects must also be considered. The thermal release of dissolved and adsorbed hydrogen occurs in the form of molecules at the wall temperatures of present devices and of plasma systems of the next generations. This leads one to expect that 1. (a) a large hydrogen concentration exists in the subsurface layers during the discharge and even a considerable time after its termination, 2. (b) pronounced memory effects should exist as evidenced during isotope changeover experiments; the behaviour should differ markedly when the second isotope is fed into the system at the start or when it is injected during the discharge. 3. (c) the hydrogen inventory in and its permeation through the wall should be larger than the value evaluated from its solubility at the filling pressure. These effects — and in particular their variation with the wall temperature and with the parameters of the impinging particles — are discussed and compared with available data obtained in tokamaks and in simulation devices. The domains of applicability of relevant computer codes are briefly addressed. The need for additional information on specific material data and existing methods to acquire them are indicated.