The increasing appropriation of advanced high-strength steels is accompanied by the requirement of enhanced formability, which could be only achieved by the implementation of certain quantities of retained austenite (RA) to generate the required TRIP-effect. However, the increasing strength level is also associated with heightened susceptibility to hydrogen embrittlement (HE). We have therefore investigated the role of RA and its connection to a transforming microstructure in hydrogen-related crack initiation and propagation in advanced high-strength steels under realistic conditions. Varying concentrations of RA have been attained through different heat treatments and advanced characterization techniques have been used. In order to study the transforming microstructure a series of different mechanical tests, including constant load test, slow strain rate tensile tests and cyclic loading tests have been performed. These investigations have revealed interesting trends. Overall, an increase in RA content results in an increased sensitivity to H embrittlement. A major insight is, however, that the details of these trends depend on the type of mechanical test and the H charging conditions. Alongside the experimental investigations, atomistic calculations have been performed on the microscopic and nanoscale features of the various materials to understand the role that RA, the neighboring microstructure and point defects have in the HE. One highlight is the ab initio investigation on the impact of H on the phase stability of austenite with respect to ferrite. These ab initio calculations serve as inputs for scale-bridging simulations at the meso- and macro-scale to understand H-related phenomena at the component-scale, connecting the microstructural characterization of the different steels with the fractography and the results obtained from cyclic loading tests. The simulations have shown in the equilibrium state there is pronounced H segregation to the microstructure regions subjected to high hydrostatic tensile stresses and plastic strains after phase transition. The multiscale modelling for the H redistribution in the transforming microstructure during mechanical strain underlines the critical role of RA.