The fuel retention in ITER will be dominated by co-deposition. Therefore predictions of retention in ITER require a global model of migration of light (Be, C) impurities and their co-deposition with hydrogenic species. To track the global material erosion/deposition balance and the resulting formation of mixed material layers the WallDYN code has been developed. This paper describes the application of WallDYN to the interpretation of results on deposition and co-deposition in JET experiments both in the ITER like wall (ILW) and full carbon configuration. The calculations show qualitative agreement with the Be deposition patterns determined from post-campaign wall tile analysis. The calculated retention results for C and Be first wall configurations even show quantitative agreement with experimental gas balance measurements when long term outgassing is taken into account. Applying WallDYN to ITER for different first wall and plasma configurations shows that for the current (Be + W only) material choice, retention will not limit ITER operation whereas C would increase retention by factors 10 to 100, leading to unacceptably high fuel retention by co-deposition.