As the final chapter, experimental limitation and remaining problems for understanding hydrogen (H) behavior and kineticsKinetics in metals are summarized. In spite of extensive studies on H behavior in metals, many unsolved or poorly understood problems are remaining in kineticsKinetics processes of H in metals, such as absorptionAbsorption, dissolutionDissolution, diffusion, trappingTrapping and permeationPermeation. In addition, models fully describing the kinetic processes have not been established yet. Two reasons could be mentioned for that, i.e. the lack of (1) detailedH concentration H concentrationConcentration profileConcentration profile (H mappingMapping or H depthProfiling profilingDepth profile) in metals and its sequential changesSequential change with time, and (2) physical and chemical stateChemical state of H accumulated at particular (trappingTrapping) sites. Although the changes of concentrations of H dissolved in grains and accumulated in trapping sitesTrapping site are strongly interrelated, due to quite large difference between them, simultaneous determination of both and observation of their sequential changesSequential change are quite difficult. They remain as important future research targets.