Our aim is to gain a detailed understanding of how human embryonic stem cells are regulated- how do embryonic stem cells decide whether to multiply to produce more stem cells, or to begin forming specialized cell types. We have found that human embryonic stem cultures are not homogeneous but are composed of different cellular subpopulations whose identities can be clearly defined at the molecular level. Only a minority of cells in the population has the capacity for self renewal, the ability to form new stem cells. This ability to divide to produce new stem cells depends on factors made by the stem cells themselves. Other cells in the culture have begun the process of specialization, with many on the way to becoming precursors of the central nervous system. Again, the choice to become a nerve cell depends on signals from surrounding cells in the culture. Understanding the conversations between subsets of stem cells is crucial to efforts to grow pure populations of stem cells or specialized cell types.