The International System of Units (SI units) forms the basis for all measurements. With the Meter Convention in 1875 a worldwide system of units was introduced with prototypes for the meter and kilogram. Even today the kilogram prototype is by definition the unit of mass. Long-term comparisons of the kilogram prototype with other mass standards indicate that the prototype is not stable with time so that the General Conference on Weights and Measures at his last meeting in November 2014 published a resolution with the recommendation to define the SI units in terms of invariants of nature. Quantum phenomena in solid state physics play a crucial role in this development. The talk summarizes the application of the quantum Hall effect in metrology with the focus on the replacement of the kilogram by a fixed value for the Planck constant.