The reduction of iron ores with carbon, such as coal and coke, generates 7-8% of the global CO2 emissions. A paradigm shift from fossil-fuel-based to green-hydrogen-based ironmaking is urgently needed to address the decarbonization challenge in this sector and combat global warming. Hydrogen-based direct reduction of iron ores is one of the most promising solutions in this context when hydrogen is produced using renewable energy. The byproduct of this reduction reaction is water. Hydrogen-based direct reduction is a multistep solid-gas reaction where the solid undergoes several non-volume conserving phase transformations and iron ores are gradually reduced into iron. At the microscopic scale, the reduction process is associated with multistep solid-gas reactions and phase transformations, mass transport through heterogeneous media, and intense mechanical interaction among the phases. This leads to the evolution of a complex defect cosmos, including vacancies, dislocations, interfaces, and free surfaces (cracks, pores), all with different transport features. These defects act as reaction, nucleation, and diffusion sites, shaping the overall reduction kinetics. This talk showcases the hierarchical microstructure formation during the hydrogen-based direct reduction and reveals its salient roles in the reduction kinetics. Particularly, the application of in-situ dark-field X-ray microscopy is highlighted to better characterize the dynamic evolution of the microstructural defects and understand the underlying reaction mechanisms.