Developmental programs are controlled by transcription factors and chromatin 
regulators, which maintain specific gene expression programs through epigenetic 
modification of the genome. These regulatory events at enhancers contribute to 
the specific gene expression programs that determine cell state and the 
potential for differentiation into new cell types. Although enhancer elements 
are known to be associated with certain histone modifications and transcription 
factors, the relationship of these modifications to gene expression and 
developmental state has not been clearly defined. Here we interrogate the 
epigenetic landscape of enhancer elements in embryonic stem cells and several 
adult tissues in the mouse. We find that histone H3K27ac distinguishes active 
enhancers from inactive/poised enhancer elements containing H3K4me1 alone. This 
indicates that the amount of actively used enhancers is lower than previously 
anticipated. Furthermore, poised enhancer networks provide clues to unrealized 
developmental programs. Finally, we show that enhancers are reset during nuclear 
reprogramming.