To understand and control hydrogen induced cracking and stress corrosion cracking, the processes of hydrogen absorption, diffusion and trapping are of interest. Fundamentals of these processes are described and of the determination of permeation coefficient, diffusivity and solubility of H in iron and steels, using the electrochemical double cell. With this method trapping parameters are also obtained, i.e. numbers of traps and binding energies. Extended studies were conducted on hydrogen in ternary alloys Fe-Me-C or N (Me = Ti, Zr, V, Nb, Mo) and in pipeline steels. Flat traps with binding energies around -19kJ/mol H can be discerned from deep traps with binding energies around -57kJ/mol H. As shown by constant extension rate tests with the pipeline steels, only the mobile hydrogen in ideal solution and in the flat traps is involved in hydrogen induced stress corrosion cracking, not the hydrogen tied up in deep traps.