Polymers. Porous organic polymers, hyper-crosslinked polymers, and polymers with intrinsic microporosity reversibly store and release hydrogen through hydrogen physisorption on their highly porous structures and in some cases use combinations of physisorption and chemisorption to store H2. The introduction of functional groups in polymer networks can also provide an enhancement of the hydrogen uptake. Porous polymers containing highly electron-deficient carborane-components were successfully used as adsorbents for hydrogen. Ketone and N-heterocycle containing polymers fixed and stored hydrogen at atmospheric pressure through the formation of chemical bonds to form the corresponding alcohol and hydrogenated N-heterocycle polymers, respectively. The hydrogenated polymers released hydrogen in the presence of catalysts at mild conditions. Quinaldine-substituted polyacrylic acid and its hydrogenated 1,2,3,4-tetrahydroquinaldine derivative reversibly release hydrogen by heating (80ºC) in the presence of an aqueous iridium complex catalyst. Conducting polymers consisting of a polyaniline matrix, which can be functionalized by catalytic doping or by the introduction of chemical groups into a polymer molecule, is a potential promise for hydrogen storage. It was reported that polyaniline could store up to 6-8 wt% of hydrogen. Porous polyaniline P-PANI facilitates hydrogen diffusion and reaction kinetics of the hydrogen storage alloys. Polyacrylamide blending with ammonia borane enables the dehydrogenation of the polymeric composite to occur at lower temperature with enhanced hydrogen purity. Although hydrogen storage by means of physisorption has some limitations, polymers seem very promising materials, due to their high potential of structural and functional tuning, as well as good thermal and chemical stability.