This talk covers a variety of ongoing efforts at the National Institute of Standards and Technology (NIST) to understand and overcome metrological challenges posed by in-process monitoring of metal additive manufacturing (AM) processes. It provides a brief overview of the Measurement Science for Additive Manufacturing (SMAS) Program, and the goals of the Metrology for Real-time Monitoring of AM project. These goals can be divided into two areas: research-based efforts to advance the current state of in-process monitoring technologies, and standardization efforts to promote principles and guidelines for those aspects of in-process monitoring that are mature enough to standardize. The talk provides a brief overview of some of NIST's standardization efforts in ASTM E07 and F42, and previews some of the NIST research aimed at identifying best methods for calibration and characterization of AM in-process monitoring systems, data management and transport, and data registration. Finally, the talk discusses current trends and activities in the areas of AM in-process monitoring, and provides some outlooks on how these technologies and associated standards may evolve.