The subdivision of proliferating tissues into groups of non-intermingling sets of cells, termed compartments, is a common process of animal development. Signaling between adjacent compartments induces the local expression of morphogens that pattern the surrounding tissue. Sharp and straight boundaries between compartments stabilize the source of such morphogens during tissue growth and, thus, are of crucial importance for pattern formation. Signaling pathways required to maintain compartment boundaries have been identified, yet the physical mechanisms that maintain compartment boundaries remained elusive. Recent data now show that a local increase in actomyosin-based mechanical tension on cell bonds is vital for maintaining compartment boundaries in Drosophila.