Hydrogen embrittlement of 316L type austenitic stainless steel has been studied by charging thin tensile specimens (0.2 mm thick) with hydrogen through cathodic polarization. The effects of hydrogen on the phase transition and the relative role of the metallurgical variables is discussed. Room temperature cathodic charging of unstressed specimens produces intergranular and secondary transgranular cracks along crystallographic planes. Severe grain boundary spelling has been observed at longer times of charging indicating that high stresses were formed. The surface cracking that was observed during the ageing is consistent with the development of high tensile surface stresses. TEM studies of the fracture surfaces of both annealed and sensitized, fine and coarse grain size, have revealed high dislocation structure. Thin plates of hydrogen inducede (h c p)-martensite was observed. These plates appear in a heavily faulted region. The evidence of faults withine-plates indicates that the overlapping stacking fault mechanism for the austenite toe transformation is in agreement with strain inducede. The results of the tensile tests while undergoing cathodic charging show that the additional sensitization treatment and coarse-grained samples, lower the mechanical properties. The fracture surfaces of the sensitized steel contains regions of intergranular fracture where the micromechanism of the failure is microvoid nucleation and coalescence along grain boundaries. Finally, the microstructures are connected to various modes of cracking.