Unleashing oil and gas resources would be greatly benefited with better understanding of the mechanisms behind hydrogen embrittlement. Carbon steel is the cost-effective materials of choice for pipelines and casings for sour service in the oil and gas industry. Hydrogen embrittlement weakens the interatomic bonds between grains of steel casing or offshore tools and causes premature failures. This phenomenon could pose a risk to the sustainability of the oil and gas projects. Application of high strength steels has been an evolving field of research in the oil and gas industry. The utilization of high strength steels in increasingly more harsh environments has promoted manufacturing of new generations of highly ductile high-strength steels. Oil and gas wells could have hydrogen sulfide and therefore hydrogen embrittlement could be triggered. Emergent intricacy in designing downhole tools safely along with requirements to assure stiffness with less wall thickness has increased the use of high strength steels. Complex Phase steels were born once researchers discovered the benefits of partially replacing martensite by bainite. The diffusion of hydrogen through various types of materials have been a prime interest of engineers and scientists in miscellaneous fields. It is well known that when atomic hydrogen diffuses into steels with various microstructures, it could lead to hydrogen embrittlement, a phenomenon which its underlying mechanisms are yet to be precisely explained. The purpose of this paper is to elucidate how hydrogen diffusion yields to hydrogen embrittlement. The mechanisms of hydrogen trapping, types of traps and the effects of lattice defects on hydrogen diffusion in addition to the classical hydrogen embrittlement theories are discussed in detail. Hydrogen diffusion in austenite, ferrite and cementite were compared and it was shown how thermal desorption analysis and permeation could determine a percolating threshold in preventing the passageway of hydrogen.