This tutorial covers the basic issues of metrology and measurement activity. The measurement concept is introduced, showing why a measurement result cannot ever be the true value of the measured quantity. The uncertainty concept is defined, and the way it can be expressed and estimated is analyzed. The recommendation of the present international reference standard (IEC ISO Guide to the Expression of Uncertainty in Measurement) is also discussed. Alessandro Ferrero is a professor of electrical and electronic measurements at the Dipartimento di Elettrotecnica of the Politecnico di Milano. His current research interests are concerned with the application of digital methods to electrical measurements and measurements on electric power systems under nonsinusoidal conditions, and with the metrological characterization of instruments based on complex DSP algorithms.