This chapter delves into the intricate interplay of measurement, decision rules, and declarations of conformity in the realm of conformity assessment. Exploring the fundamental components, the chapter elucidates the pivotal role of measurement accuracy and reliability in ensuring products, processes, and systems meet established standards. The discussion navigates through the complexities of measurement uncertainty, highlighting its inherent nature and essentiality in decision-making processes. Decision rules, acting as guiding principles, are explored with real-world examples from pharmaceutical manufacturing and environmental compliance. These examples underscore the criticality of precise measurements in conformity assessment, ensuring compliance and safeguarding public well-being. The chapter emphasizes the pragmatic application of rigorous standards, accurate measurements, and well-understood uncertainties in the declaration of conformity. By embracing emerging technologies and upholding ethical considerations, the chapter envisions a future where accurate measurements stand as the cornerstone of conformity assessment, fostering a world where quality knows no compromise.