Physics describes objects and phenomena through certain properties, which are called “physical quantities”. These physical quantities must be quantifiable, for exampel through a measurement process, in which case we speak of a “measured quantity”. To meaningfully state the value of a measured quantity, a numerical value, also called a magnitude, and a unit must be given. In this chapter we introduce the modern system of units with some reference to historical units. It is based on a series of basic quantities, from which the other physical quantities are derived. We use the basic quantities and units of the Système International d'unités, the SI-system. We introdcue the basic quantities and their units.