Around the same time that the first sequence of the full human genome was being announced by President Bill Clinton at the White House, an arguably bigger and more ambitious task was being launched in Sweden. The Human Protein Atlas (HPA) promised to catalog every protein in the human body, providing the information completely free through an open-access database. Twenty years on, the HPA has made huge strides thanks to hard work, new technologies, and the dedication of hundreds of researchers. In this expansive digital supplement to Science, we take the reader on a journey through the milestones reached by the HPA and the research it has generated. We hope this publication will be a useful reference, while also giving readers a sense of the enormity of the task undertaken by the HPA, the extraordinary advances that have resulted from it, and the crucial information this database provides for researchers everywhere.