1. Infancy: all the bone marrow is haemopoietic
2. Childhood: progressive fatty replacement
3. Adult: confined to central skeleton and proximal ends of femurs and humeri
4. 50% fat even in these areas
5. Capable of reverting to haemopoiesis
6. Liver and spleen can resume fetal haemopoietic role (‘extramedullary haemopoiesis’)