Units are at the heart of science and technology. Most measurements require units to express their result. The International System of Units (SI) provides a widely used coherent framework of units. While the present SI is fit for purpose for all measurements, it has several shortcomings. Most notably, the kilogram is defined via an artifact that limits its realization to a specific place and certain times. To solve this problem and to take advantage of advances in quantum metrology made in the last 50 years, a revision of the SI is planned. This revision will modify the foundation of the SI from base units to fundamental constants of nature. While the revision presents a huge shift in philosophy, the size of the units will not change, and hence the revision will go mostly unnoticed.