The human immune system is extremely complex, comprised of multiple cell types and states interacting in myriad ways to produce diverse cellular ecosystems. The rise of single-cell genomics in recent years has contributed a great deal to understanding this complexity and the role of the immune system in infection, inflammation, and disease. In this seminar Muzlifah Haniffa will demonstrate the application of single-cell genomics to decode the developing human immune system. In particular, she will discuss her work using single-cell RNA sequencing to study human yolk sac, fetal liver and bone marrow haematopoiesis and the immune network formation in prenatal peripheral tissues. Muzlifah will discuss this work within her broader research goal of understanding how developmental immune programs may be co-opted in post-natal disease . A detailed understanding of the developing immune system is also relevant to improve stem cell therapy and regenerative medicine.