Hematopoiesis – the formation of blood cellular components – occurs during embryonic development and throughout adulthood to produce and replenish the blood system. Studying hematopoiesis can help scientists and clinicians to understand better the processes behind blood disorders and cancers. Furthermore, hematopoietic stem cells (HSCs) can be used as a model system for understanding tissue stem cells and their role in ageing and oncogenesis. In this article, and in the accompanying poster, we provide an overview of the process of hematopoiesis, highlighting the sites of hematopoiesis in various organisms, and the factors that regulate HSC emergence and self-renewal.