**1. Introduction**

Metrology is the science of measurements. In our everyday life, we are constantly surrounded by measurements: from reading the time to weighing apples, we continuously measure something. However, measurements are also somehow embedded in objects, since, for example, the apple we buy has already been measured before its arrival at the greengrocer, in order to determine its calibre. In these measurements, uncertainty plays a very important rule. Metrologists know that no measurement makes sense without an associated uncertainty value. Without it, no decision can be taken; no comparisons can be made; no conformity can be assessed.

It is hence pivotal to know the meaning of measurement uncertainty, understand the contributions to measurement uncertainty, know how these contributions affect the final measurement uncertainty, have a mathematical tool to represent measurement uncertainty and propagate it through the measurement procedure, and consider measurement uncertainty in any application.

This Topical Collection "Measurement Uncertainty" started as a Special Issue, but many contributions have been submitted showing how metrology—and, in particular, measurement uncertainty—is an open, interesting, and important topic.

Therefore, with my great pleasure, the Special Issue has become a Topical Collection. I invite Colleagues working on this issue to continue submitting papers, so that the Collection can grow and become a good place for a fruitful discussion.
