The concepts of traceability and uncertainty are foundational in the field of metrology to ensure measurement accuracy and reliability. This chapter explores these concepts by emphasizing the importance of calibration hierarchies in establishing traceability. A concrete example is presented using dosimetry protocols for calibration of radiation therapy treatment devices, where traceability ensures accurate absorbed dose measurements. We describe the roles of international standards and regulatory bodies in harmonizing global measurement systems to facilitate trade and ensure quality infrastructure. Through examination of primary standard dosimetry methods, including calorimetry and ionometry, this chapter details the critical role of standards laboratories in maintaining metrological integrity.