Belle II is the natural continuation of the Belle and BaBar experiments started about 20 years ago at the KEKB (Tsukuba, Japan) and PEP-II (Stanford,U SA) B-factories, respectively. These two experiments have discovered CP violation in the B meson system, confirmed the Cabibbo-Kobayashi-Maskawa quark mixing mechanism and completed an extensive programme of physical measurements of flavour, by studying both particles containing beauty or charm quarks as well as τ leptons. To date, all the measurements taken are compatible with the Standard Model; yet we know that this is not able to explain all of the phenomena observed, such as the baryon asymmetry of the universe or the mass of neutrinos; it is therefore important to find new physics to indicate how the Standard Model should be overcome. Precision measurements of rare processes in flavour physics constitute an important tool to achieve this goal, and the electron-positron colliders have proven to be a very valuable, and in some cases, the only tool. For this reason, Japan decided to fund a substantial improvement of the KEKB accelerator, with the SUPER-KEKB project; the goal is to reach a luminosity 40 times greater and in five years provide a number of events 50 times the database accumulated by BELLE and BABAR. The Belle II detector is, in turn, an upgrade of Belle, in particular with the substitution of the tracking system, improvement of the particle detectors and the increased capacity to support the more difficult experimental conditions due to the increased luminosity of the machine.