The pillar for outreach in particle physics, which is nowadays an integral part of our work as researchers, is IPPOG, the International Particle Physics Outreach Group. IPPOG is a network of scientists, science educators and communication specialists working across the globe in informal science education and public engagement for particle physics. The flagship activity of IPPOG is the International Particle Physics Masterclasses programme, to which other activities such as the Worldwide Data Day, the International Muon Week and International Cosmic Day organisation have been added. IPPOG members also participate in a wide range of events: public talks, festivals, exhibitions, teacher training, student competitions, and open days at local institutes. A resource database has also been developed containing a wealth of material for the dissemination of particle physics. In this paper the history and evolution of IPPOG is presented briefly, and its various activities are discussed, with emphasis on the masterclasses, which have been expanding both geographically and in scope during the years.