Directed energy deposition (DED) additive manufacturing techniques have advantages for producing large-scale metallic components because of its fast deposition rate, high feedstock efficiency, and low manufacturing costs. DED processes employ highly concentrated energy sources to rapidly melt metallic feedstocks, subjecting materials to rapid cooling and thermal cycling throughout the layer-by-layer manufacturing process. Such unique thermal conditions generate microstructures rarely achieved through conventional manufacturing processes, such as element segregation, high dislocation density, and microstructure heterogeneity. These distinctive structure features present challenges for simulating the microstructures in DED materials. Although post-mortem characterizations can yield high-resolution high-fidelity results, they cannot reveal the transient structure dynamics present during the DED process, thereby offering limited mechanism-based guidance for further developing the numerical models. We have been studying the wire-laser DED processes of structural alloys (e.g., Inconel 718, 316L stainless steel) using operando synchrotron X-ray techniques. Full-field X-ray imaging provided an overview of the printing process, as well as direct observation of the morphological change of the feedstock wire and melt pool upon laser heating. X-ray diffraction, on the other hand, was used to characterize the melting state of the feedstock wire, the solidification behavior, phase transformations, and the evolution of dislocation density. The quantitative structural information measured in operando X-ray experiments can be directly used to calibrate and validate multiphysics and microstructure models. More importantly, the critical phenomena revealed in X-ray experiments offer significant implications for developing new models that capture all the physics controlling microstructure development in DED materials.