During their studies or vocational training, natural scientists, engineers and technicians usually learn how to correctly use physical quantities, units and equations. In their professional lives, however, this often fades into the background. This paper, which will refresh this knowledge and serve as a reference, provides an overview of the relevant international standards. It exclusively deals with the units of the International System of Units and the quantities of the International System of Quantities. It describes the units and quantities that can be used in accordance with the SI or ISQ and the logarithmic quantities with decibel.