The U.S. National Institute of Standards and Technology (NIST) has launched the Real-Time Monitoring (RTM) for Additive Manufacturing project, aimed at developing the metrological science foundation needed for in-situ monitoring of metal additive manufacturing processes. The project focuses on measurement methods for process signatures at the melt pool and layer scales, and establishes calibration and characterization techniques for monitoring tools, providing a basis for potential standardization. The core of the project lies in using industrial-grade monitoring systems to generate large, inter-comparable reference datasets of controlled process monitoring, exploring the correlation between sensor signals and part quality. Current commercial systems have integrated melt pool monitoring (MPM) and layer-wise monitoring (LM) technologies, which can observe melt pool temperature, geometry, or powder layer condition, but sensor design, image analysis and signal processing specifications, and instrument calibration remain incomplete. The project also addresses low-technology-readiness methods such as acoustic monitoring and multispectral optical emission measurement, striving to overcome obstacles such as the high heterogeneity of monitoring instruments and the lack of standard protocols. NIST explicitly states that accessible process measurement and control methods will help industry build, qualify, and certify parts at higher throughput rates, accelerating the adoption of additive manufacturing parts in high-performance fields such as aerospace, defense, and biomedical applications. The metrological methods and standards established by the project directly serve the improvement of quality control systems for metal additive manufacturing processes.