In this document, after a review of the accidents/incidents, are described the different interactions between hydrogen gas and the most commonly used materials including the influence of "internal" and "external" hydrogen, the phenomena occurring in all ranges of temperatures and pressures and Hydrogen Embrittlement (HE) created by gaseous hydrogen.The principle of all the test methods used to investigate this phenomenon are presented and discussed. The advantages and disadvantages of each method will be explained.The document also covers the influence of all the parameters related to HE including the ones related to the material itself , the ones related to the design and manufacture of the equipment and the ones related to the hydrogen itself (pressure, temperature, purity, etc.).Finally recommendations to avoid repetition of accidents/incidents mentionned before are proposed.