The Human Cell Atlas Project is an international collaborative effort aimed at defining all human cell types based on their molecular profiles and connecting this information with classical cellular descriptions. The project seeks to create a comprehensive reference map of the molecular state of cells in healthy human tissues, which would advance the systematic study of physiological states, developmental trajectories, regulatory circuits, and cellular dysregulation in diseases. The article discusses the potential utility of the Human Cell Atlas, early proofs-of-concept, and design considerations, emphasizing the importance of open data, code, and community engagement. It highlights the challenges and opportunities in characterizing cell types and states, and outlines the steps needed to achieve a detailed understanding of human cells. The project is seen as a crucial step in advancing our knowledge of biology and medicine, providing valuable tools for research and clinical applications.The Human Cell Atlas Project is an international collaborative effort aimed at defining all human cell types based on their molecular profiles and connecting this information with classical cellular descriptions. The project seeks to create a comprehensive reference map of the molecular state of cells in healthy human tissues, which would advance the systematic study of physiological states, developmental trajectories, regulatory circuits, and cellular dysregulation in diseases. The article discusses the potential utility of the Human Cell Atlas, early proofs-of-concept, and design considerations, emphasizing the importance of open data, code, and community engagement. It highlights the challenges and opportunities in characterizing cell types and states, and outlines the steps needed to achieve a detailed understanding of human cells. The project is seen as a crucial step in advancing our knowledge of biology and medicine, providing valuable tools for research and clinical applications.