High strength Nickel alloys are widely used in subsea and downhole O&G applications for their excellent combination of mechanical properties, toughness and corrosion resistance in sour environments. The trend in the O&G industry is for using higher strength materials for high pressure – high temperature service, however as strength increases materials may also become more susceptible to ambient temperature failure mechanisms associated with hydrogen absorption. In recent years resistance to Hydrogen Stress Cracking and Hydrogen Embrittlement as well as sour corrosion resistance have become of increasing interest to the industry due to a number of reported failures of high strength precipitation hardened Nickel alloy grades used in completion tools. The failures of the materials have been attributed to unfavourable microstructures increasing their susceptibility to HSC. A number of factors are known to influence HSC1,2 and this paper reviews previous work and shows laboratory results using different test techniques to demonstrate the influence of yield strength and microstructure on the resistance to Hydrogen Embrittlement of Nickel alloys 718 and alloy 945X (UNS N09946). Tests have been conducted in Hydrogen charging environments while conducting a slow strain rate test and also by pre-charging tensile samples and assessing the effect of hydrogen absorption on the materials' ductility. The resistance of the Nickel alloys to HSC is correlated with microstructure and mechanical properties.