This thesis explores microstructural damage in steels as a result of mechanical loading, hydrogen charging into the microstructure, and H2S-based corrosion with a view to informing and improving understanding of steels used in sour (H2S) service. Interstitial Free (IF) steel samples had their microstructures altered and were mechanically tested to reveal different Lüders band formation and this was correlated both with Ti-based precipitate distributions and populations with Energy Dispersive Spectroscopy (EDS) and with local Geometrically Necessary Dislocation (GND) content using High Angular Resolution Electron Backscatter Diffraction (HR-EBSD). Four-point bend coupons of Q125R and L80 (quenched and tempered martensitic steels) were deformed and electrolytically charged with hydrogen. Three crack geometries were observed: (1) Hydrogen Induced Crack (HICs); (2) Stress Orientation Hydrogen Induced Cracks (SOHICs); (3) Sulphide Stress Cracks (SSCs). Combined EDS and EBSD revealed that HICs were seen to initiate regardless of bending stress and were correlated with sulphur-rich regions of the microstructure. Conversely, no correlation of SOHICs and SSCs with sulphur-rich regions of the microstructure was found. The extent to which SSCs propagate along Prior Austenite Grain (PAG) boundaries was also quantified using EBSD. Finally, wet H2S autoclave testing on four-point bend coupons of Q125R and L80 was performed. Three tests were conducted at H2S partial pressures of 0.1 bar(a), 0.5 bar(a) and 1.0 bar(a). No HICs, SOHICs or SSCs developed in the 0.1 bar(a) test. However, microstructural damage sites as a result of hydrogen egress was detected around large sulphur-inclusions in the microstructure. Macroscopic cracking developed in all Q125R samples in the 0.5 bar(a) and 1.0 bar(a) tests. Little to no microscopic damage or cracking was found in the L80 samples. These findings are drawn together to discuss the relative role of microstructural features on damage accumulation and failure of steels for use in sour gas applications.