The micro-mechanisms of hydrogen embrittlement become active once the local hydrogen concentration is sufficiently high. Ab-initio methods are employed to obtain insights into the segregation and diffusion of hydrogen in iron, in the presence of microstructural features such as vacancies, simple representative grain boundaries, and various transition metal carbides and their interfaces with the matrix. The results will be used to inform a Kinetic Monte Carlo-based diffusion model for hydrogen. The diffusion coefficients thus obtained can be incorporated into macro-scale diffusion models.