The aim of this dissertation is to investigate the effect of interfaces on phase transformation, yielding behaviour, and hydrogen embrittlement resistance, which can provide bottom-up guidance to design medium-Mn steels (MMnS) with desired mechanical properties. In order to obtain qualitative and quantitative information on microstructure and nanostructure, multi-scale characterization techniques were utilized in combination with thermodynamics-based simulation in the present work. Electron backscattering diffraction (EBSD), high-energy synchrotron diffraction (SYXRD), and transmission electron microscopy measurements were applied to study the microstructural evolution. Atom probe tomography (APT) was dedicatedly used to deliver chemical information at various interfaces and to analyse the elemental partitioning of solute elements at the near-atomic scale. To evaluate hydrogen-induced mechanical degradation and hydrogen concentration, cathodic hydrogen charging, thermal desorption analysis (TDA), and slow strain rate tensile (SSRT) testing were implemented. Additionally, the thermodynamics-based diffusion module (DICTRA) was utilized to predict the formation of concentration profiles at migrating interfaces and correlated with experimental observation. By engineering the interfacial features, their interplay with dislocations and consequently the mechanical properties can be controlled. This study emphasizes the significance and potential of interface engineering in tailoring the microstructural evolution and in tuning mechanical properties of MMnS. In the foreseeable future, interface engineering may contribute to the design of high-performance, sustainable steels for the automotive, energy, and offshore industry.