Understanding and controlling microstructure evolution in metal additive manufacturing (AM) is essential for producing reliable, certifiable parts. However, the highly nonequilibrium conditions inherent to AM, such as rapid heating and cooling rates and cyclic thermal histories, make it difficult to perform real-time structural characterization and to understand the in situ response of AM materials to post-processing. To address this, we have developed a suite of metrology tools at NIST under the framework of Structural Metrology of Advanced Manufacturing Processes to measure the evolution of material structure in situ across multiple time and length scales. This presentation highlights recent advances in high-brightness, high-energy synchrotron X-ray diffraction and scattering methods tailored to probe transient phase transformations and structural kinetics during laser-based AM and post-build heat treatments. By integrating complementary methods, including multi-modal electron microscopy and computational thermodynamic and thermokinetic modelling, we have uncovered previously unresolved transformation sequences and explained the formation kinetics of unexpected phases under specific thermal histories. These insights have enabled the development of mitigation strategies through process control and post-build treatments. Our metrology has been applied to a wide range of metal systems and has informed critical benchmark datasets for the AM modelling community, including those used in AM Bench challenges. By providing foundational, high-fidelity structural data, this work supports the calibration of simulation tools and accelerates the pathway toward certifiable AM components across industries.