The Human Cell Atlas aims to chart the types and properties of all human cells, across all tissues and organs, to build a reference map of the healthy human body. The human body is made of trillions of cells – the fundamental units of life – which divide, grow and acquire distinct functions in the embryo, eventually leading to different cell types (such as skin cells, neurons or fat cells) that form the various tissues of the body. These tissues come together to form organs such as the lungs and the brain. Previous knowledge of cells has come from looking at them under a microscope, or more recently by analysing clumps of hundreds or thousands of cells and finding the average properties. However, to see the true picture for every cell type, it is necessary to first separate the cells and then find out what molecules are produced in each. These molecules include sets of RNA messages, called the transcriptome, which help give each cell its own identity and distinguish it from the many other cell types found in the body. A few years ago, measuring this complex and extensive information would have been impossible, but recent technological advances in the field of single-cell genomics can separate individual cells from different tissues and organs, and measure the transcriptome or other important molecules from each of them.