This article, the tenth in a series on the biological basis of child health, focuses on blood. Blood has a crucial role in the transport of substances such as respiratory gases, nutrients and antibodies, as well as in acid-base balance, fluid balance, blood clotting and the immune system. This article describes the composition, formation and function of blood, outlines normal blood count values and explains the effects of low blood cell counts in children. It also provides an overview of the blood disorders that are commonly seen in children, including anaemia, sickle cell disease, clotting disorders and blood cancers. It is essential for children's nurses to have knowledge and an understanding of blood, including its physiology and pathophysiology, to provide optimal care and support to children and young people and their families.