The 37 trillion cells of the human body have a remarkable array of specialised functions, and must cooperate and collaborate in time and space to construct a functioning human. In this talk I will describe my lab’s efforts to understand this cellular diversity through a programme of cell atlasing. Harnessing cutting edge single cell genomics, imaging and computational technologies, we investigate development, homeostasis and disease states, at scale and in 3D, with a particular focus on immunity. I will illustrate the relevance of cell atlas-ing for engineering organoids and regenerative medicine, and will share new results providing insights into pacemaker cells from the sinoatrial node of this heart. Overall I hope to illustrate the power of single cell approaches in unlocking fundamental knowledge about the human body.