In May 2019, the new SI definitions for amps, kilograms, kelvins and moles came into force. For the mole, the SI unit of the amount of substance, the Avogadro constant was redefined and its value was set as the defining constant. But: How did it come about that a comparative number became a natural constant? Embark on a short journey from the beginnings to the present day: From the gas laws of the 17th century to the new SI system of units of the 21st century. Get to know the meaning of the natural constants in general and the Avogadro constant in particular. Get an overview of the term “mole” and find out which (exemplary) methods can be used to determine the Avogadro constant.