I am very pleased to invite you to attend the Quadrennial Ozone Symposium (QOS) in South Korea, on October 3 (Sunday) - 9 (Saturday), 2021. Despite its tiny fraction, ozone is a crucial gas in the Earth Atmosphere. The ozone layer in the stratosphere protects life on Earth by filtering out damaging ultraviolet radiation from the sun. In the troposphere, ozone is an important greenhouse gas and a strong pollutant at ground level. Since the last QOS that took place in Edinburgh in 2016, a variety of new developments have taken place on all facets of atmospheric ozone, including its observation and impact on human health and ecosystems. The Symposium’s program addresses all these aspects. Through the foreseen keynote talks, presentations at the sessions and discussions at poster sessions and side events, QOS 2021 is of high interest for all scientists throughout the world interested in atmospheric ozone and more generally in the study of the Earth’s atmosphere.