Along the hydrogen supply chain, metallic components, such as pressure vessels, compressors and valves, are facing high pressure hydrogen gas. The object of this paper is to address microstructural as well as mechanical aspects of fatigue crack initiation and growth at room temperature in a quenched and tempered (Q&T) low alloy steel under hydrogen pressure in the range 0.5-35 MPa. For such steel, the need to perform tests insitu under hydrogen pressure is required. The influence of hydrogen gas on the total life in terms of crack initiation and crack propagation is analyzed. The experimental techniques developed to detect crack initiation in a pressure vessel under hydrogen pressure are presented. Thanks to these technical developments the influence of hydrogen gas on the total life duration including crack initiation and crack propagation is analyzed. It is shown that the effect of hydrogen pressure on crack initiation is important. At constant load ratio, the hydrogen pressure effect on fatigue crack growth (FCG) is dependent on the loading amplitude (in terms of Delta K). These results related to cracking behavior are enriched with information on fracture surfaces appearance. The results presented have been achieved within the European project MATHRYCE [1] dedicated to Material Testing and Recommendations for Hydrogen Components under fatigue. They are part of a process necessary to give a scientific background to the development of a design methodology where hydrogen enhanced fatigue damage is taken into account. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.