Bacillus subtilis is the best studied of the bacteria that make heat-resistant spores in response to starvation. The process of sporulation results in the formation of a cell type which is quite different in morphology from that of bacteria during normal growth. Sporulation requires the activation, in an ordered sequence, of many genes that are kept silent during vegetative growth. It also requires that these genes be activated differentially in two sister cells that are genetically identical. The sophisticated mechanisms responsible for both the temporal and the spatial regulation of gene expression are now understood in a fair amount of detail. They are likely to provide models which will make a valuable contribution to studies of development and differentiation in higher cells.