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 promised to catalog every protein in the human body, providing the information completely free through an open-access database. Twenty years on, huge strides have been made 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 and the research published. We hope that this publication will be a useful reference, while also giving readers a sense of the enormity of the task undertaken, the extraordinary advances that have resulted from it, and the crucial information that this database provides for researchers everywhere.