The flagship activity of the International Particle Physics Outreach Group (IPPOG) is International Masterclasses (IMC) in particle physics. This very successful programme brings cutting-edge science to high-school students. Invited to a university or laboratory, the students spend a day of immersion in particle physics, learning about the Standard Model, open research questions, experimental methods, detectors, and accelerators. The hands-on part involves analysis of data from an experiment. The IMC programme started in Europe, was later joined by America, and has now expanded to all continents. The data used were initially from LEP; with the turn-on of the LHC, measurements with data from ALICE, ATLAS, CMS and LHCb were introduced. In recent years, there has been a spectacular broadening of the physics content of the IMC, with data from Belle II at KEK, from neutrino experiments (MINERvA and NOvA at Fermilab), from astroparticle physics (Pierre Auger), and medical applications (Particle Therapy).