This critical review covers the mechanisms underlying density functional theory (DFT) simulations and their relevance in evaluating, developing and discovering new materials. It is intended to be of interest for both experimentalists and theorists in the expanding field of hydrogen storage. We focus on the most studied classes of materials, metal-hydride, -amide, and -borohydride mixtures, and bare and transition metal-doped carbon systems and the utility of DFT simulations for the pre-screening of thermally destabilised reaction paths (170 references).