<p>Environmental sciences are supported by multi-decadal observations from instruments on land, in the oceans, in the air, and from space. For such observations to have sufficient quality to underpin decisions, they need the long-term stability and global equivalence achieved through traceability to an internationally agreed reference, preferably the international system of units, the SI. Such traceability is achieved through metrological principles of linking measurements through an unbroken chain of comparisons to suitable references, with rigorous uncertainty analysis following internationally accepted approaches, and validated through comparisons with independent approaches at key steps of the chain. Increasingly, communities responsible for coordinating observational networks and transforming their data into products and services, whether from satellites or other platforms, are calling for metrological principles to be applied across the entire value chain. However, it is not always straightforward to apply these principles to observations of the Earth’s environmental system. This paper discusses what these metrological principles are, how they differ from other approaches in use in the observational sciences, and how the modern, evolving metrological guides can be used by those observational communities, to meet the challenging requirements of climate-quality observations and the data products derived from them.</p>