The elaboration of pattern within insect segments is a well-studied example of cellular patterning during development. This process requires that each cell develop appropriately for its position. Experimental embryology suggests that intercellular communication plays a key role in imparting positional information to cells. Drosophila genetics has identified numerous genes whose activity is required for patterning within segments, and whose molecular genetic analyses suggest they constitute and control cell communication circuits. Particular genes are expressed or required by cells that will follow distinct developmental pathways, and some appear to confer or interpret intercellular signals. Other patterning genes are ubiquitously required and may provide the machinery through which the signals are transmitted.