ACP was launched in September 2001 as the first open-access journal with open peer review in its field. To celebrate its success and immense growth since then, we compile in this special issue a collection of opinion articles to reflect on the developments of the field of atmospheric chemistry and physics in the last 20 years and point to exciting directions for the future, which will stimulate the scientific debate in our field for the next 20 years. Only invited contributions to the special issue will be considered, and the review process will be overseen by the ACP executive and senior editors.