The Human Protein Atlas is a comprehensive database that provides the protein expression profiles for a large number of human proteins, presented as immunohistological images from most human tissues. This review provides an overview of the contents of the atlas, discusses the project strategy and highlights the importance of open access for data validation and quality. Essential procedures that are implemented during antibody production and image generation, such as the use of protein epitope signature tags (PrEST) antigens, monospecific antibodies, tissue microarrays and thorough quality validation, are also discussed. The Human Protein Atlas is related to four other expression atlas initiatives, including, in particular, an upcoming protein atlas developed by the Sanger Institute.