Many current and next generation energy systems are reliant on the production, transportation, storage and use of gaseous hydrogen, often at high pressure. The safety, durability, performance, and economic operation of such systems are challenged due to the reality that hydrogen promotes a variety of degradation modes in otherwise high performance materials. Such degradation is often manifested as cracking which compromises the structural integrity of metals and polymers; a behaviour complicated by time and operating cycle (e.g., stress, hydrogen pressure, and temperature) dependencies of degradation. As an example, concurrent stressing and hydrogen exposure at typical pressure vessel or pipeline environmental conditions can promote cracking in modern metallic systems at one-tenth the fracture toughness. Such hydrogen-induced degradation phenomena are generally categorised as hydrogen embrittlement. The breadth and importance of hydrogen damage phenomena have not gone unnoticed in the scientific community with an immense amount of work conducted over the past 100 years. The problem is broadly interdisciplinary and such work has involved metallurgy, chemistry, solid mechanics and fracture mechanics, surface science, molecular and atomic hydrogen physics, non-destructive inspection, materials characterisation, and mechanical-properties testing. This important work notwithstanding, major challenges face those tasked with managing complex engineering structures exposed to demanding environment and mechanical loading conditions. The challenge here is to transform debate on mechanisms of hydrogen damage into a focus on quantitative, predictive models of material cracking properties. Overarching these challenges is the inescapable fact that hydrogen damage problems are immensely complex, requiring understanding of time-cycle dependent processes operating at the atomic scale to impact behaviour manifest at the macroscopic scale.