Human pluripotent stem cells are defined by their abilities to multiply indefinitely in the laboratory and to turn into any type of body cell. However, stem cell cultures are not composed of one cell type, but are heterogeneous. In this study, we took a close up look at human embryonic stem cell cultures by examining the properties of individual single cells, rather than studying the population as a whole. The result show that the population of cells in the culture dish comprises a complex hierarchy, from primitive cells with great capacity for multiplication through to cells just on the verge of becoming specialized cell types. Specific signals govern how cells at different stages of this hierarchy behave: whether they divide to form more stem cells, or begin to specialize into different cell types such as nerve cells. The cells themselves produce signals that govern how they move through this hierarchy. By understanding these internal dialogues between different populations in stem cell culture, we can learn how to control stem cell behaviour, and to understand why some stem cell lines respond differently to others under specific culture conditions. In turn, this knowledge will enhance our ability to propagate stem cells and to turn them into particular cell types useful in research or regenerative medicine.