Measurement Uncertainty This reprint focuses on a very important topic in metrology which is represent by measurement uncertainty. Any good metrologist or scientist in engineering knows that no measurement makes sense without an associated uncertainty value: without an uncertainty value no decision can be taken; no comparisons can be made; no conformity can be assessed. Any decision comparison or conformity assessment made without considering the measurement uncertainty affecting the measurement value is completely useless and meaningless. Stated that it becomes very clear that uncertainty in measurement plays indeed a very important rule in our everyday life. This is the reason why there is a great need to have a fruitful academic and scientific discussion on this topic. We have been speaking about measurement uncertainty for less than 30 years since the concept of “measurement uncertainty” has been introduced in 1995 by the “Guide to the expression of uncertainty in measurement” (GUM). Thirty years seems to be many but still the concept of measurement uncertainty has not been spread worldwide and the GUM is a document that is not known everywhere. On the other hand this document should be considered not only in academic scenario but also in any technical and industrial scenario where it is pivotal to know the meaning of measurement uncertainty identify the uncertainty contributions and know how these contributions affect the final measurement result